EXPERIMENTAL: Metal backend (#441)

This is not a continuation of the Metal backend; this is simply bringing
the branch up to date and merging it as-is behind an experiment.

---------

Co-authored-by: Isaac Marovitz <isaacryu@icloud.com>
Co-authored-by: Samuliak <samuliak77@gmail.com>
Co-authored-by: SamoZ256 <96914946+SamoZ256@users.noreply.github.com>
Co-authored-by: Isaac Marovitz <42140194+IsaacMarovitz@users.noreply.github.com>
Co-authored-by: riperiperi <rhy3756547@hotmail.com>
Co-authored-by: Gabriel A <gab.dark.100@gmail.com>

src/Ryujinx.Graphics.Metal/Effects/IPostProcessingEffect.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace Ryujinx.Graphics.Metal.Effects
+{
+    internal interface IPostProcessingEffect : IDisposable
+    {
+        const int LocalGroupSize = 64;
+        Texture Run(Texture view, int width, int height);
+    }
+}