Add the SDL_GPU API

Project Lead: Evan Hemsley <evan@moonside.games>

Co-designer, Metal Port, Console Ports:

Co-authored-by: Caleb Cornett <caleb.cornett@outlook.com>

Production, QA, Debug:

Co-authored-by: Ethan Lee <flibitijibibo@gmail.com>

SDL_Render Driver, Bugfixes:

Co-authored-by: Andrei Alexeyev <akari@taisei-project.org>

Additional D3D12 Programming, Bugfixes:

Co-authored-by: Bart van der Werf <bluelive@gmail.com>

Bugfixes and Feedback:

Co-authored-by: Zakary Strange <zakarystrange@gmail.com>
Co-authored-by: meyraud705 <meyraud705@gmail.com>
Co-authored-by: Joshua T. Fisher <playmer@gmail.com>
Co-authored-by: Topi Ritala <ritalat@fastmail.com>
Co-authored-by: David Gow <david@ingeniumdigital.com>

Original API Proposal:

Co-authored-by: Ryan C. Gordon <icculus@icculus.org>

test/testgpu/cube.metal

@@ -0,0 +1,38 @@
+#include <metal_stdlib>
+using namespace metal;
+
+struct VSOutput
+{
+    float4 color [[user(locn0)]];
+    float4 position [[position]];
+};
+
+#ifdef VERTEX
+
+struct UBO
+{
+    float4x4 modelViewProj;
+};
+
+struct VSInput
+{
+    float3 position [[attribute(0)]];
+    float3 color [[attribute(1)]];
+};
+
+vertex VSOutput vs_main(VSInput input [[stage_in]], constant UBO& ubo [[buffer(0)]])
+{
+    VSOutput output;
+    output.color = float4(input.color, 1.0);
+    output.position = ubo.modelViewProj * float4(input.position, 1.0);
+    return output;
+}
+
+#else
+
+fragment float4 fs_main(VSOutput input [[stage_in]])
+{
+    return input.color;
+}
+
+#endif