Tech ENABLE_GLBEGIN_GLEND_REMOVAL - Background rendering

(cherry picked from commit prusa3d/PrusaSlicer@eda55701a2)

resources/shaders/background.fs

@@ -0,0 +1,11 @@
+#version 110
+
+uniform vec4 top_color;
+uniform vec4 bottom_color;
+
+varying vec2 tex_coord;
+
+void main()
+{
+    gl_FragColor = mix(bottom_color, top_color, tex_coord.y);
+}

resources/shaders/background.vs

@@ -0,0 +1,9 @@
+#version 110
+
+varying vec2 tex_coord;
+
+void main()
+{
+    gl_Position = gl_Vertex;
+	tex_coord = gl_MultiTexCoord0.xy;
+}