Add standalone testbed

imgui/StandaloneImGuiTestbed/Shaders/SPIR-V/generate-spirv.bat

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+glslangvalidator -V imgui-vertex.glsl -o imgui-vertex.spv -S vert
+glslangvalidator -V imgui-frag.glsl -o imgui-frag.spv -S frag

imgui/StandaloneImGuiTestbed/Shaders/SPIR-V/imgui-frag.glsl

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+#version 450
+
+#extension GL_ARB_separate_shader_objects : enable
+#extension GL_ARB_shading_language_420pack : enable
+
+layout(set = 1, binding = 0) uniform texture2D FontTexture;
+layout(set = 0, binding = 1) uniform sampler FontSampler;
+
+layout (location = 0) in vec4 color;
+layout (location = 1) in vec2 texCoord;
+layout (location = 0) out vec4 outputColor;
+
+void main()
+{
+    outputColor = color * texture(sampler2D(FontTexture, FontSampler), texCoord);
+}

imgui/StandaloneImGuiTestbed/Shaders/SPIR-V/imgui-vertex.glsl

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+#version 450
+
+#extension GL_ARB_separate_shader_objects : enable
+#extension GL_ARB_shading_language_420pack : enable
+
+layout (location = 0) in vec2 in_position;
+layout (location = 1) in vec2 in_texCoord;
+layout (location = 2) in vec4 in_color;
+
+layout (binding = 0) uniform ProjectionMatrixBuffer
+{
+    mat4 projection_matrix;
+};
+
+layout (location = 0) out vec4 color;
+layout (location = 1) out vec2 texCoord;
+
+out gl_PerVertex
+{
+    vec4 gl_Position;
+};
+
+void main() 
+{
+    gl_Position = projection_matrix * vec4(in_position, 0, 1);
+    color = in_color;
+    texCoord = in_texCoord;
+}