Add standalone testbed

imgui/StandaloneImGuiTestbed/Shaders/Metal/imgui-frag.metal

@@ -0,0 +1,18 @@
+#include <metal_stdlib>
+using namespace metal;
+
+struct PS_INPUT
+{
+    float4 pos [[ position ]];
+    float4 col;
+    float2 uv;
+};
+
+fragment float4 FS(
+    PS_INPUT input [[ stage_in ]],
+    texture2d<float> FontTexture [[ texture(0) ]],
+    sampler FontSampler [[ sampler(0) ]])
+{
+    float4 out_col = input.col * FontTexture.sample(FontSampler, input.uv);
+    return out_col;
+}