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/Shaders/StencilBlit.metal

@@ -0,0 +1,27 @@
+#include <metal_stdlib>
+
+using namespace metal;
+
+struct CopyVertexOut {
+    float4 position [[position]];
+    float2 uv;
+};
+
+struct Textures
+{
+    texture2d<uint, access::sample> texture;
+    sampler sampler;
+};
+
+struct FragmentOut {
+    uint stencil [[stencil]];
+};
+
+fragment FragmentOut fragmentMain(CopyVertexOut in [[stage_in]],
+                             constant Textures &textures [[buffer(TEXTURES_INDEX)]]) {
+    FragmentOut out;
+
+    out.stencil = textures.texture.sample(textures.sampler, in.uv).r;
+
+    return out;
+}