ASTC optimizations (#845)

* ASTC optimizations

* Move code to Ryujinx.Common

* Support 3D textures

* Address feedback

* Remove ASTC logging

* Use stackalloc instead of a Buffer20 struct

* Code style and cleanup

* Respond to feedback

* Rearrange public/private property ordering

Ryujinx.Common/Utilities/Buffers.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Diagnostics;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+namespace Ryujinx.Common.Utilities
+{
+    [DebuggerDisplay("{ToString()}")]
+    [StructLayout(LayoutKind.Sequential, Size = 16)]
+    public struct Buffer16
+    {
+        [DebuggerBrowsable(DebuggerBrowsableState.Never)] private ulong _dummy0;
+        [DebuggerBrowsable(DebuggerBrowsableState.Never)] private ulong _dummy1;
+
+        public byte this[int i]
+        {
+            get => Bytes[i];
+            set => Bytes[i] = value;
+        }
+
+        public Span<byte> Bytes => SpanHelpers.AsByteSpan(ref this);
+
+        // Prevent a defensive copy by changing the read-only in reference to a reference with Unsafe.AsRef()
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static implicit operator Span<byte>(in Buffer16 value)
+        {
+            return SpanHelpers.AsByteSpan(ref Unsafe.AsRef(in value));
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static implicit operator ReadOnlySpan<byte>(in Buffer16 value)
+        {
+            return SpanHelpers.AsReadOnlyByteSpan(ref Unsafe.AsRef(in value));
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public ref T As<T>() where T : unmanaged
+        {
+            if (Unsafe.SizeOf<T>() > (uint)Unsafe.SizeOf<Buffer16>())
+            {
+                throw new ArgumentException();
+            }
+
+            return ref MemoryMarshal.GetReference(AsSpan<T>());
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public Span<T> AsSpan<T>() where T : unmanaged
+        {
+            return SpanHelpers.AsSpan<Buffer16, T>(ref this);
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public readonly ReadOnlySpan<T> AsReadOnlySpan<T>() where T : unmanaged
+        {
+            return SpanHelpers.AsReadOnlySpan<Buffer16, T>(ref Unsafe.AsRef(in this));
+        }
+    }
+}