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.

src/compton.h

@@ -740,10 +740,11 @@ presum_gaussian(conv *map);
 
 static XImage *
 make_shadow(Display *dpy, double opacity,
-            int width, int height);
+            int width, int height, Bool clear_shadow);
 
 static Picture
-shadow_picture(Display *dpy, double opacity, int width, int height);
+shadow_picture(Display *dpy, double opacity, int width, int height,
+    Bool clear_shadow);
 
 static Picture
 solid_picture(Display *dpy, Bool argb, double a,
@@ -922,6 +923,9 @@ set_focused(Display *dpy, win *w, Bool focused) {
 static void
 determine_fade(Display *dpy, win *w);
 
+static void
+win_update_shape(Display *dpy, win *w);
+
 static void
 determine_shadow(Display *dpy, win *w);