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);
+}

resources/shaders/imgui.vs

@@ -0,0 +1,17 @@
+#version 110
+
+uniform mat4 ProjMtx;
+
+attribute vec2 Position;
+attribute vec2 UV;
+attribute vec4 Color;
+
+varying vec2 Frag_UV;
+varying vec4 Frag_Color;
+
+void main()
+{
+	Frag_UV = UV;
+	Frag_Color = Color;
+    gl_Position = ProjMtx * vec4(Position.xy, 0.0, 1.0);
+}