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.Shader/CodeGen/Msl/HelperFunctions/SwizzleAdd.metal

@@ -0,0 +1,7 @@
+float swizzleAdd(float x, float y, int mask, uint thread_index_in_simdgroup)
+{
+    float4 xLut = float4(1.0, -1.0, 1.0, 0.0);
+    float4 yLut = float4(1.0, 1.0, -1.0, 1.0);
+    int lutIdx = (mask >> (int(thread_index_in_simdgroup & 3u) * 2)) & 3;
+    return x * xLut[lutIdx] + y * yLut[lutIdx];
+}