Optimize texture format conversion, and MethodCopyBuffer (#1274)

* Improve performance when converting texture formats.

Still more work to do.

* Speed up buffer -> texture copies.

No longer copies byte by byte. Fast path when formats are identical.

* Fix a few things, 64 byte block fast copy.

* Spacing cleanup, unrelated change.

* Fix base offset calculation for region copies.

* Fix Linear -> BlockLinear

* Fix some nits. (part 1 of review feedback)

* Use a generic version of the Convert* functions rather than lambdas.

This is some real monkey's paw shit.

* Remove unnecessary span constructor.

* Revert "Use a generic version of the Convert* functions rather than lambdas."

This reverts commit aa43dcfbe8.

* Fix bug with rectangle destination writing, better rectangle calculation for linear textures.

Ryujinx.Graphics.Texture/Bpp12Pixel.cs

@@ -0,0 +1,11 @@
+using System.Runtime.InteropServices;
+
+namespace Ryujinx.Graphics.Texture
+{
+    [StructLayout(LayoutKind.Sequential, Pack = 1, Size = 12)]
+    public struct Bpp12Pixel
+    {
+        private ulong _elem1;
+        private uint _elem2;
+    }
+}