Minor cleanup

Variable was never used. Clean up both C++ and shader code.

Incoming texture is UNORM, so the value must already be saturated.

Lambda replacement was added, but old function was left behind.

Shuts down browser panels early to ensure that cookie data doesn't
necessarily get lost.

UI: Remove 5 file limit for drag & drop

(This commit also modifies the UI, obs-ffmpeg, and obs-output modules)

Fixes a long-time regression where the program would lock up if an
encode call fails.  Shuts down all outputs associated with the failing
encoder and displays an error message to the user.

Ideally, it would be best if a more detailed error could be displayed to
the user about the nature of the error, though the primary problem is
the encoder errors are typically not something the user would be able to
understand.  The current message is a bit of a generic error message;
improvement is welcome.

Another suggestion is to try to have the encoder restart seamlessly,
though it would take a significant amount of work to be able to make it
do something like that properly, and it sort of assumes that encoder
failures are sporadic, which may not necessarily be the case with some
hardware encoders on some systems.  It may be better just to use another
encoder in that case.  For now, seamless restart is ruled out.

UI: Dark theme consistency for Scenes vs Sources

- Selected but unfocused Source colour
- Scenes font
- Padding when editing Source name

UI: Fix hotkey theme issues

UI: Add hover color to hotkey icons

libobs/UI: Add Transition Previews

This commit adds a preview to the properties window for transitions.

The preview will play back the transition at the global transition
duration or the transitions fixed duration, between two private scenes
with an A and B label, and different background colors.

This commit adds a function to forcefully stop a transition, and to
increment/decrement the showing counter for a source with the MAIN_VIEW
type.

These functions are needed for the transition previews to work as
intended.

Closes obsproject/obs-studio#1639

Fix VAAPI CBR

libobs: UI: Remove Qt usage from graphics thread

libobs: Restore casts to fix Clang warnings

obs-outputs: Fix undefined MSG_NOSIGNAL

This fixes Xcode warning about detecting UI usage from another thread.

Define MSG_NOSIGNAL even if CRYPTO is not defined.

Clang doesn't like it if you convert T** to const T* const* without
casting even though it should be safe. Might be a C language thing.

Avoid sigpipe on writing to closed file descriptors

obs-frontend-api: Add method to trigger a Studio Mode transition

obs-qsv: Fix QSV failing with new DCH drivers

obs-frontend-api: Add methods to get/set transition duration

This patch fixes QSV failing with new DCH graphics drivers. QSV is not
initializing under certain conditions when the graphics adapter 0 is not
set to iGFX due to outdated MSDK dispatcher. Updating to MSDK with new
dispatcher to enable proper initialization of QSV encoder with DCH
drivers.

This can happen when trying to write to a closed socket (in the case
of an RTMP server closing the connection).

This could happen if writing to a socket which was closed by the peer.

libobs: libobs-d3d11: Fix alpha blend

There are cases where alpha is multiplied unnecessarily. This change
attempts to use premultiplied alpha blending for composition.

To keep this change simple, The filter chain will continue to use
straight alpha. Otherwise, every source would need to modified to output
premultiplied, and every filter modified for premultiplied input.

"DrawAlphaDivide" shader techniques have been added to convert from
premultiplied alpha to straight alpha for final output. "DrawMatrix"
techniques ignore alpha, so they do not appear to need changing.

One remaining issue is that scale effects are set up here to use the
same shader logic for both scale filters (straight alpha - incorrectly),
and output composition (premultiplied alpha - correctly). A fix could be
made to add additional shaders for straight alpha, but the "real" fix
may be to eliminate the straight alpha path at some point.

For graphics, SrcBlendAlpha and DestBlendAlpha were both ONE, and could
combine together to form alpha values greater than one. This is not as
noticeable of a problem for UNORM targets because the channels are
clamped, but it will likely become a problem in more situations if FLOAT
targets are used.

This change switches DestBlendAlpha to INVSRCALPHA. The blending
behavior of stacked transparents is preserved without overflowing the
alpha channel.

obs-transitions: Use premultiplied alpha blend, and simplify shaders
because both inputs and outputs use premultiplied alpha now.

Fixes https://obsproject.com/mantis/view.php?id=1108

libobs: Add additional effect debugging information