Add support for the R4G4 texture format (#2956)

Ryujinx.Graphics.Texture/PixelConverter.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Runtime.InteropServices;
+using System.Runtime.Intrinsics;
+using System.Runtime.Intrinsics.X86;
+
+namespace Ryujinx.Graphics.Texture
+{
+    public static class PixelConverter
+    {
+        public unsafe static byte[] ConvertR4G4ToR4G4B4A4(ReadOnlySpan<byte> data)
+        {
+            byte[] output = new byte[data.Length * 2];
+            int start = 0;
+
+            if (Sse41.IsSupported)
+            {
+                int sizeTrunc = data.Length & ~7;
+                start = sizeTrunc;
+
+                fixed (byte* inputPtr = data, outputPtr = output)
+                {
+                    for (ulong offset = 0; offset < (ulong)sizeTrunc; offset += 8)
+                    {
+                        Sse2.Store(outputPtr + offset * 2, Sse41.ConvertToVector128Int16(inputPtr + offset).AsByte());
+                    }
+                }
+            }
+
+            Span<ushort> outputSpan = MemoryMarshal.Cast<byte, ushort>(output);
+
+            for (int i = start; i < data.Length; i++)
+            {
+                outputSpan[i] = (ushort)data[i];
+            }
+
+            return output;
+        }
+    }
+}