Fix blending issue with invert effect

Actually it's more a hack and should be addressed in the next
release cycle by allowing effects to modify the scene's blend
equation.
BUG: 274424
FIXED-IN: 4.7.0

effects/invert/data/invert.frag

@@ -23,6 +23,7 @@ void main()
 
     tex *= modulation;
     tex.rgb = vec3(1.0) - tex.rgb;
+    tex.rgb *= tex.a;
 
     gl_FragColor = tex;
 }