Add standalone testbed

imgui/StandaloneImGuiTestbed/Shaders/GLSL/imgui-vertex.glsl

@@ -0,0 +1,20 @@
+#version 330 core
+
+uniform ProjectionMatrixBuffer
+{
+    mat4 projection_matrix;
+};
+
+in vec2 in_position;
+in vec2 in_texCoord;
+in vec4 in_color;
+
+out vec4 color;
+out vec2 texCoord;
+
+void main()
+{
+    gl_Position = projection_matrix * vec4(in_position, 0, 1);
+    color = in_color;
+	texCoord = in_texCoord;
+}