Add standalone testbed

imgui/StandaloneImGuiTestbed/Shaders/HLSL/imgui-frag.hlsl

@@ -0,0 +1,15 @@
+struct PS_INPUT
+{
+    float4 pos : SV_POSITION;
+    float4 col : COLOR0;
+    float2 uv  : TEXCOORD0;
+};
+
+Texture2D FontTexture : register(t0);
+sampler FontSampler : register(s0);
+
+float4 FS(PS_INPUT input) : SV_Target
+{
+    float4 out_col = input.col * FontTexture.Sample(FontSampler, input.uv);
+    return out_col;
+}

imgui/StandaloneImGuiTestbed/Shaders/HLSL/imgui-vertex.hlsl

@@ -0,0 +1,27 @@
+cbuffer ProjectionMatrixBuffer : register(b0)
+{
+    float4x4 ProjectionMatrix;
+};
+
+struct VS_INPUT
+{
+    float2 pos : POSITION;
+    float2 uv  : TEXCOORD0;
+    float4 col : COLOR0;
+};
+
+struct PS_INPUT
+{
+    float4 pos : SV_POSITION;
+    float4 col : COLOR0;
+    float2 uv  : TEXCOORD0;
+};
+
+PS_INPUT VS(VS_INPUT input)
+{
+    PS_INPUT output;
+    output.pos = mul(ProjectionMatrix, float4(input.pos.xy, 0.f, 1.f));
+    output.col = input.col;
+    output.uv = input.uv;
+    return output;
+}