(This commit also modifies UI)

Removes obs_scene::group_sceneitem and replaces it with
obs_scene::is_group.  Changes a number of other functions related to
groups so that a group is not inherently tied to a specific scene, and
helps allow a single group to be referenced in multiple scenes if
desired.

Removes a likely unintentional self-comparison.

As declared, transform_val is static inline, but it should be marked as
a static inline void since it does not return.

Allows the ability to group scene items.  Groups internally are
sub-scenes, which allows the ability to add unique filters and
transforms to each group.

Allows scenes to have a custom size.  Mainly used for grouping, but can
be extended with public API later.

Prevents a potential race condition with the scene mutex.

Defers updating the scene item transform to the graphics thread to
reduce the number of transform update calls.

Fixes a bug where items that had cropping would recalculate transform
every frame.  obs_scene_item::last_width and obs_scene_item::last_height
would be set to the cropped sizes rather than the source's actual size.

Allows using obs_scene_add_internal to add without sending signals.

When a scene is added as a scene item with the same audio sources that
are already in the current scene, it would cause the current scene to no
longer output audio due to audio.

To replicate the issue, you would create two separate audio device
captures in scene 1, use add existing in scene 2 and add one of those
audio sources, then go back to scene 1, add scene 2 as a source, then
make scene 1 invisible.

Scene filters were not being copied when a scene was duplicated with
obs_scene_duplicate.

Closes jp9000/obs-studio#1104

Ensures that private settings data is also duplicated when a scene or a
source is duplicated.

Allows setting and sharing of user data for sources and scene items.

Fixes https://obsproject.com/mantis/view.php?id=508

The toggle_visibility set only by init_hotkeys() call, but used each
time in obs_sceneitem_destroy() call.  Because zero hotkey_pair_id is
valid - we need to set item property other than zero here.

Closes jp9000/obs-studio#1000

When item in the old scene had only Scale Filter, texture renderer
was not created for an item in the new scene. However if the item also
had Crop set, the texture renderer was created.

Now after copying the data, texture renderer is created for the item in
the new scene when needed.

Closes jp9000/obs-studio#969

Scene items would incorrectly have pre-multiplied alpha when they were
scaled with different scale filtering, when they were cropped, or when
the item itself was a scene.  This happens because the scene renders the
items to a texture in those cases, and when they are rendered to a
texture the blend function would be the default srcalpha/invsrcalpha
blend function, which would cause alpha to become pre-multiplied in the
texture's result rather than straight alpha.

This changes the behavior to directly copy the color/alpha to the
texture using the one/zero blend function instead, which makes the
resulting texture straight alpha.  (Note that you do not want to turn
off the blend mode for the same result because certain sources can have
custom drawing that may rely on blending being available)

Related Issue: https://obsproject.com/mantis/view.php?id=954

Closes jp9000/obs-studio#966

Adds a lock checkbox to allow the user to lock a specific scene item.

Closes jp9000/obs-studio#949

Adds a method of uniquely identifying scene items, as well as finding
those scene items via that unique ID.

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.