Tech ENABLE_GL_IMGUI_SHADERS - Render imgui using shaders

(cherry picked from commit prusa3d/PrusaSlicer@d0d89a4d5b)

resources/shaders/imgui.fs

@@ -0,0 +1,11 @@
+#version 110
+
+uniform sampler2D Texture;
+
+varying vec2 Frag_UV;
+varying vec4 Frag_Color;
+
+void main()
+{
+	gl_FragColor = Frag_Color * texture2D(Texture, Frag_UV.st);
+}