Prevents infinite recursion when adding children to a scene.

When duplicating scenes, scene item scale filtering options would not be
preserved.

When there are audio sources in a scene and they've all stopped playing
their audio (audio is pending), all scene item audio actions (volume
changes, toggling visibility) will perpetually buffer and no longer be
processed until audio plays again.

So instead of that, if all audio sources have stopped playing in the
scene, just process all pending scene item audio actions immediately to
prevent them from never being processed while waiting for a scene item
to start playing audio.

Allows rendering scenes as scene items without off-screen sub-source of
the scene popping out on the sides.

Saves the scene item scale filters as strings instead of enums.  It's
important to save data in a form that won't break if an API enum
changes.

Allows the ability to use scale filters such as point, bicubic, lanczos
on specific scene items, disabled by default.  When using one of the
latter two options, if the item's scale is under half of the source's
original size, it uses the bilinear low resolution downscale shader
instead.

The crop texture would not always render correctly because the
projection matrix had not been set.

Renders the scene item to a texture if crop is enabled; otherwise
renders directly.

Allows the ability to defer updating the item transform until all
functions have been executed.

Makes signals use stack memory rather than allocate memory each time.
Most likely a completely insignificant and pointless optimization.

A comment that serves as a reminder to anyone who might need to edit the
scene code.  If the graphics mutex must be locked, it must be locked
first before entering the scene mutexes, or outside of the scene
mutexes.

(Note: This commit also modifies the UI)

Allows the ability to duplicate sources fully copied, and/or have the
scene and its duplicates be private sources

Creates a scene marked as a private source

Prevents a mutual lock with the scene mutex and graphics mutex.  In
libobs/obs-video.c, the graphics mutex could be locked first, then the
scene mutexes second, while in the UI thread, the scene mutexes could be
locked first, then when a scene item is being destroyed, a source could
be destroyed, and sometimes sources would lock the graphics mutex
second.

A possible additional solution is to defer source destroys to the video
thread.

Helps identify a scene without having to compare its type name to
"scene".

(Note: test and UI are also modified by this commit)

API Changed (removed "enum obs_source_type type" parameter):
-------------------------
obs_source_get_display_name
obs_source_create
obs_get_source_output_flags
obs_get_source_defaults
obs_get_source_properties

Removes the "type" parameter from these functions.  The "type" parameter
really doesn't serve much of a purpose being a parameter in any of these
cases, the type is just to indicate what it's used for.

This buffers scene item visibility actions so that if
obs_sceneitem_set_visible to true or false, that it will ensure that the
action is mapped to the exact sample point time in which
obs_sceneitem_set_visible is called.  Mapping to the exact sample point
isn't necessary, but it's a nice thing to have.

Mark scenes as composite sources and use audio_render to mix and output
the audio of their sub-sources.

Splits the scene mutex in to an audio/video mutex to allow video/audio
thread to use the same data at the same time without locking on each
other.

Before if a source was set to invisible it would still be considered
active.  This changes it so that the source is deactivated when the
source is invisible to reduce needless resource usage or capturing.

Renames:
----------------------------------------
obs_source_add_child
obs_source_remove_child
obs_source_enum_sources
obs_source_enum_tree
obs_source_info::enum_sources

To:
----------------------------------------
obs_source_add_active_child
obs_source_remove_active_child
obs_source_enum_active_sources
obs_source_enum_active_tree
obs_source_info::enum_active_sources

These functions/callbacks had misleading names: they originally implied
any child sources, when they actually meant active child sources that
are being used to render video or audio.  It's important that the
function names represent their actual purpose.

API changed from:
obs_source_info::get_name(void)
obs_output_info::get_name(void)
obs_encoder_info::get_name(void)
obs_service_info::get_name(void)

API changed to:
obs_source_info::get_name(void *type_data)
obs_output_info::get_name(void *type_data)
obs_encoder_info::get_name(void *type_data)
obs_service_info::get_name(void *type_data)

This allows the type data to be used when getting the name of the
object (useful for plugin wrappers primarily).

NOTE: Though a parameter was added, this is backward-compatible with
older plugins due to calling convention.  The new parameter will simply
be ignored by older plugins, and the stack (if used) will be cleaned up
by the caller.

Instead of manually setting the blend state to the desired values, use
gs_reset_blend_state to ensure we have the default blend state (which
for color is a typical srcalpha/invsrcalpha alpha blending operation,
then the alpha channels are added together).

This fixes an issue where filtered scenes would look strange due to the
fact that alpha was not being blended properly.

This is just in case a filter is applied to a scene, filters will turn
off the blending so it's necessary to set the blending.

Allows setting a specific position for scene items rather than just
up/down/top/bottom

These signals introduce unnecessary complexity.  Instead of emitting a
signal for a specific move direction, just signal that the scene has
been reordered, that way the target just refreshes the list.

Previously a NULL item would cause a crash when reading transform info.
The crash can be reproduced by e.g. editing scenes.json so that two
scenes contain each other. Example scenes.json:

{
    "current_scene": "Scene",
    "sources": [
        {
            "flags": 0,
            "id": "scene",
            "mixers": 0,
            "name": "Scene",
            "settings": {
                "items": [
                    {
                        "align": 5,
                        "bounds": {
                            "x": 1440.0,
                            "y": 900.0
                        },
                        "bounds_align": 0,
                        "bounds_type": 2,
                        "name": "Scene 2",
                        "pos": {
                            "x": 0.0,
                            "y": 0.0
                        },
                        "rot": 0.0,
                        "scale": {
                            "x": 1.0,
                            "y": 1.0
                        },
                        "visible": true
                    }
                ]
            },
            "sync": 0,
            "volume": 1.0
        },
        {
            "flags": 0,
            "id": "scene",
            "mixers": 0,
            "name": "Scene 2",
            "settings": {
                "items": [
                    {
                        "align": 5,
                        "bounds": {
                            "x": 1.0,
                            "y": 1.0
                        },
                        "bounds_align": 0,
                        "bounds_type": 0,
                        "name": "Scene",
                        "pos": {
                            "x": 854.0,
                            "y": -520.0
                        },
                        "rot": 0.0,
                        "scale": {
                            "x": 1.75,
                            "y": 1.7562724351882935
                        },
                        "visible": true
                    }
                ]
            },
            "sync": 0,
            "volume": 1.0
        }
    ]
}