Fix division by zero error in shader

Author: maiself

extension.js

@@ -37,7 +37,9 @@ const InvertWindowEffect = new Lang.Class({
 			uniform sampler2D tex; \
 			void main() { \
 				vec4 color = texture2D(tex, cogl_tex_coord_in[0].st); \
-				color.rgb /= color.a; \
+				if(color.a > 0.0) { \
+					color.rgb /= color.a; \
+				} \
 				color.rgb = vec3(1.0, 1.0, 1.0) - color.rgb; \
 				color.rgb *= color.a; \
 				cogl_color_out = color * cogl_color_in; \