UI: Fix size of output icon

CONTRIBUTING.rst: Improve commit guidelines

Few words about the commit's titles were added (capital letter).

UI: Fix remux dialog ignoring filename changes

UI: Show a message in the empty source list

Simplify YUV conversion, and direct YUV444P support

Use libobs for YUV444P conversion previously handled by FFmpeg. This
ensures the libobs path resists rot.

Currently several shaders need "DrawMatrix" techniques to support the
possibility that the input texture is a "YUV" format. Also, "DrawMatrix"
is overloaded for translation in both directions when it is written for
RGB to "YUV" only.

A cleaner solution is to handle "YUV" to RGB up-front as part of format
conversion, and ensure only RGB inputs reach the other shaders. This is
necessary to someday perform correct scale filtering without the cost of
redundant "YUV" conversions per texture tap.

A necessary prerequisite for this is to add conversion support for
VIDEO_FORMAT_I444, and that is now in place. There was already a hack in
place to cover VIDEO_FORMAT_Y800. All other "YUV" formats already have
conversion functions.

"DrawMatrix" has been removed from shaders that only supported "YUV" to
RGB conversions. It still exists in shaders that perform RGB to "YUV"
conversions, and the implementations have been sanitized accordingly.

UI: Add null check for rename of default theme

Without this, we strcmp a null pointer if no theme is set in the
configuration file.

image-source: Set default size of color source to canvas size

Remove double spaces from localization

UI: Add ability to center items vertically/horizontally

UI: Show correct version in about dialog

UI: Rename Default theme to System

The system theme was named Default even though the default theme is Dark.
This addresses that by renaming Default.qss to System.qss. I've made it
backwards compatible so users already using this theme are not affected.

The theme list now shows up as:
-System
-Dark (Default)
-Acri
-Rachni

I have also made it so that you can specify the default theme in the
UI config file.

UI: Autoselect text on scene/source rename

UI: Log group items on startup

Add D3D/GL debug markers to make RenderDoc captures easier to tranverse.

Also add obs_source_get_name_no_null() to avoid boilerplate for safe
string formatting.

Closes obsproject/obs-studio#1799

Add support for debug markers via D3DPERF API and KHR_debug. This makes
it easier to understand RenderDoc captures.

D3DPERF is preferred to ID3DUserDefinedAnnotation because it supports
colors. d3d9.lib is now linked in to support this.

This feature is disabled by default, and is controlled by
GS_USE_DEBUG_MARKERS.

From: obsproject/obs-studio#1799

We can't compare addresses of ComPtr for self-reference directly because
the address-of operator is overloaded, causing a compiler error. This
fix more or less matches the WRL implementation.

UI, libobs, text-freetype2: Add missing pragma once in header files

obs-frontend-api: Access system tray icon from API

UI: Don't update stats dock if hidden

UI: Fix incorrect padding usage in Rachni theme

When the stats dock is hidden, the QTimer is still updating it, causing
unnecessary CPU usage.

UI: Fix bandwidth test flag being saved to stream key

Previously enabling and disabling bandwidth testing in a certain order
lead to the parameter being included in the stream key that is saved to
disk, making it impossible to disable the bandwidth test later on.

By doing this properly and using string concatenation instead of
modifying (appending to) the key_ value this will no longer happen.

libobs: Fix and simplify Area scale filter

It appears there's a projection flip that is applied in some situations,
like the preview pane in studio mode, and the shader math fails when
it's active causing the output color to be zero. This fixes the math for
GLSL (with a tiny redundancy penalty to HLSL), and cleans up some
unnecessary code along the way.

Use abs() to avoid zero area in case the OpenGL projection flip is
active. Also simplify the math, and remove the unnecessary sampler
state.