Add standalone testbed

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

@@ -0,0 +1,27 @@
+#include <metal_stdlib>
+using namespace metal;
+
+struct VS_INPUT
+{
+    float2 pos [[ attribute(0) ]];
+    float2 uv [[ attribute(1) ]];
+    float4 col [[ attribute(2) ]];
+};
+
+struct PS_INPUT
+{
+    float4 pos [[ position ]];
+    float4 col;
+    float2 uv;
+};
+
+vertex PS_INPUT VS(
+    VS_INPUT input [[ stage_in ]],
+    constant float4x4 &ProjectionMatrix [[ buffer(1) ]])
+{
+    PS_INPUT output;
+    output.pos = ProjectionMatrix * float4(input.pos.xy, 0.f, 1.f);
+    output.col = input.col;
+    output.uv = input.uv;
+    return output;
+}