Improvement: Change clear_shadow implementation

- Implement clear_shadow with painting region limitation instead of
  calculation in shadow image, to make it work correctly on windows with
  rounded corners, requested by funeral1988. This might cause more load
  on CPU, but could mean less load for GPU. The original implementation
  is kept but commented out.

- Code cleanup.

compton.sample.conf

@@ -12,7 +12,7 @@ shadow-offset-y = -7;
 # shadow-blue = 0.0;
 shadow-exclude = [ "n:e:Notification" ];
 # shadow-exclude = "n:e:Notification";
-shadow-ignore-shaped = true;
+shadow-ignore-shaped = false;
 
 # Opacity
 menu-opacity = 0.8;
@@ -36,7 +36,7 @@ detect-client-opacity = true;
 refresh-rate = 0;
 vsync = "none";
 dbe = false;
-paint-on-overlay = true;
+paint-on-overlay = false;
 sw-opti = false;
 
 # Window type settings