Fix setting background_opacity causing window margins/padding to be slightly different shade from background

Fixes #1221

docs/changelog.rst

@@ -3,6 +3,12 @@ Changelog
 
 |kitty| is a feature full, cross-platform, *fast*, GPU based terminal emulator.
 
+0.13.2 [future]
+------------------------------
+
+- Fix setting background_opacity causing window margins/padding to be slightly
+  different shade from background (:iss:`1221`)
+
 0.13.1 [2018-12-06]
 ------------------------------
 

kitty/cell_fragment.glsl

@@ -101,7 +101,7 @@ void main() {
 #else
     // SIMPLE
 #ifdef TRANSPARENT
-    final_color = alpha_blend_premul(fg.rgb, fg.a, background * bg_alpha, bg_alpha);
+    final_color = alpha_blend_premul(fg.rgb, fg.a, background, bg_alpha);
     final_color = vec4(final_color.rgb, final_color.a);
 #else
     // since background alpha is 1.0, it is effectively pre-multiplied