nvdec: Adding Vp8 codec support (#2707)

* first try

* second try

* working update

* Final impl

* Fixes nits

* Fix everything

* remove leftover

* Update FFmpegContext.cs

* Update Surface.cs

* Addresses gdkchan feedback

* bool not byte

* Addresses gdkchan feedback

Ryujinx.Graphics.Nvdec.FFmpeg/H264/Decoder.cs

@@ -0,0 +1,56 @@
+using FFmpeg.AutoGen;
+using Ryujinx.Graphics.Video;
+using System;
+
+namespace Ryujinx.Graphics.Nvdec.FFmpeg.H264
+{
+    public sealed class Decoder : IH264Decoder
+    {
+        public bool IsHardwareAccelerated => false;
+
+        private const int WorkBufferSize = 0x200;
+
+        private readonly byte[] _workBuffer = new byte[WorkBufferSize];
+
+        private FFmpegContext _context = new FFmpegContext(AVCodecID.AV_CODEC_ID_H264);
+
+        private int _oldOutputWidth;
+        private int _oldOutputHeight;
+
+        public ISurface CreateSurface(int width, int height)
+        {
+            return new Surface(width, height);
+        }
+
+        public bool Decode(ref H264PictureInfo pictureInfo, ISurface output, ReadOnlySpan<byte> bitstream)
+        {
+            Surface outSurf = (Surface)output;
+
+            if (outSurf.RequestedWidth != _oldOutputWidth ||
+                outSurf.RequestedHeight != _oldOutputHeight)
+            {
+                _context.Dispose();
+                _context = new FFmpegContext(AVCodecID.AV_CODEC_ID_H264);
+
+                _oldOutputWidth = outSurf.RequestedWidth;
+                _oldOutputHeight = outSurf.RequestedHeight;
+            }
+
+            Span<byte> bs = Prepend(bitstream, SpsAndPpsReconstruction.Reconstruct(ref pictureInfo, _workBuffer));
+
+            return _context.DecodeFrame(outSurf, bs) == 0;
+        }
+
+        private static byte[] Prepend(ReadOnlySpan<byte> data, ReadOnlySpan<byte> prep)
+        {
+            byte[] output = new byte[data.Length + prep.Length];
+
+            prep.CopyTo(output);
+            data.CopyTo(new Span<byte>(output).Slice(prep.Length));
+
+            return output;
+        }
+
+        public void Dispose() => _context.Dispose();
+    }
+}