NVDEC implementation using FFmpeg (#443)

* Initial nvdec implementation using FFmpeg

* Fix swapped channels on the video decoder and the G8R8 texture format

* Fix texture samplers not being set properly (regression)

* Rebased

* Remove unused code introduced on the rebase

* Add support for RGBA8 output format on the video image composer

* Correct spacing

* Some fixes for rebase and other tweaks

* Allow size mismatch on frame copy

* Get rid of GetHostAddress calls on VDec

Ryujinx.Graphics/VDec/FFmpegFrame.cs

@@ -0,0 +1,14 @@
+namespace Ryujinx.Graphics.VDec
+{
+    unsafe struct FFmpegFrame
+    {
+        public int Width;
+        public int Height;
+
+        public byte* LumaPtr;
+        public byte* ChromaBPtr;
+        public byte* ChromaRPtr;
+
+        public byte[] Data;
+    }
+}