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/Vic/StructUnpacker.cs

@@ -0,0 +1,69 @@
+using Ryujinx.Graphics.Memory;
+using System;
+
+namespace Ryujinx.Graphics.Vic
+{
+    class StructUnpacker
+    {
+        private NvGpuVmm Vmm;
+
+        private long Position;
+
+        private ulong Buffer;
+        private int   BuffPos;
+
+        public StructUnpacker(NvGpuVmm Vmm, long Position)
+        {
+            this.Vmm      = Vmm;
+            this.Position = Position;
+
+            BuffPos = 64;
+        }
+
+        public int Read(int Bits)
+        {
+            if ((uint)Bits > 32)
+            {
+                throw new ArgumentOutOfRangeException(nameof(Bits));
+            }
+
+            int Value = 0;
+
+            while (Bits > 0)
+            {
+                RefillBufferIfNeeded();
+
+                int ReadBits = Bits;
+
+                int MaxReadBits = 64 - BuffPos;
+
+                if (ReadBits > MaxReadBits)
+                {
+                    ReadBits = MaxReadBits;
+                }
+
+                Value <<= ReadBits;
+
+                Value |= (int)(Buffer >> BuffPos) & (int)(0xffffffff >> (32 - ReadBits));
+
+                BuffPos += ReadBits;
+
+                Bits -= ReadBits;
+            }
+
+            return Value;
+        }
+
+        private void RefillBufferIfNeeded()
+        {
+            if (BuffPos >= 64)
+            {
+                Buffer = Vmm.ReadUInt64(Position);
+
+                Position += 8;
+
+                BuffPos = 0;
+            }
+        }
+    }
+}