Tech ENABLE_GL_SHADERS_ATTRIBUTES - Added shaders for glsl version 140

(cherry picked from commit prusa3d/PrusaSlicer@76d1d4949b)

resources/shaders/140/background.fs

@@ -0,0 +1,11 @@
+#version 140
+
+uniform vec4 top_color;
+uniform vec4 bottom_color;
+
+in vec2 tex_coord;
+
+void main()
+{
+    gl_FragColor = mix(bottom_color, top_color, tex_coord.y);
+}