Improvement: Split shadow_pict to shadow_pict & shadow_alpha_pict

Split w->shadow_pict to w->shadow_pict and w->shadow_alpha_pict, so that
the whole w->shadow_pict need not to be rebuild on shadow opacity
change. This greatly reduces CPU usage of compton when a window with
shadow is fading. (My test shows the CPU usage of compton process
dropped from 1.15% to 0.35% when constantly fading in and out a window.)
It uses a rather painful and slow method in shadow_picture() to get
around the limitation of PictStandardA8 to make colored shadows work. I
wonder if there's a better approach.

- Merge variables gsize and cgsize as they seemingly represent the same
  thing.

src/compton.h

@@ -228,9 +228,10 @@ typedef struct _win {
   int shadow_width;
   /// Height of shadow. Affected by window size and commandline argument.
   int shadow_height;
-  /// Alpha mask Picture to render shadow. Affected by window size and
-  /// shadow opacity.
+  /// Picture to render shadow. Affected by window size.
   Picture shadow_pict;
+  /// Alpha mask Picture to render shadow. Affected by shadow opacity.
+  Picture shadow_alpha_pict;
 
   // Dim-related members
   /// Whether the window is to be dimmed.