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/ColorClear.metal

@@ -0,0 +1,38 @@
+#include <metal_stdlib>
+
+using namespace metal;
+
+struct VertexOut {
+    float4 position [[position]];
+};
+
+struct ClearColor {
+    FORMAT4 data;
+};
+
+struct ConstantBuffers {
+    constant ClearColor* clear_color;
+};
+
+vertex VertexOut vertexMain(ushort vid [[vertex_id]]) {
+    int low = vid & 1;
+    int high = vid >> 1;
+
+    VertexOut out;
+
+    out.position.x = (float(low) - 0.5f) * 2.0f;
+    out.position.y = (float(high) - 0.5f) * 2.0f;
+    out.position.z = 0.0f;
+    out.position.w = 1.0f;
+
+    return out;
+}
+
+struct FragmentOut {
+    FORMAT4 color [[color(COLOR_ATTACHMENT_INDEX)]];
+};
+
+fragment FragmentOut fragmentMain(VertexOut in [[stage_in]],
+                                  constant ConstantBuffers &constant_buffers [[buffer(CONSTANT_BUFFERS_INDEX)]]) {
+    return {constant_buffers.clear_color->data};
+}