Feature #16: _COMPTON_SHADOW window property support

- Add support for reading _COMPTON_SHADOW property from windows
  (--respect-attr-shadow). Presently the only defined behavior is, if
  _COMPTON_SHADOW is set on the highest ancestor below root window of a
  window (usually the WM frame), it's format is 32-bit, type is CADINAL,
  value is 0, the window will not get a shadow. The format and behavior
  could change in the future without prior notification.

- Fix an issue in fork_after() that may cause some streams to remain
  open. My mistake.

- Attempt to reduce determine_shadow() calls from map_win() by
  separating some raw handler functions out.

src/compton.h

@@ -318,6 +318,9 @@ typedef struct _win {
   Picture shadow_pict;
   /// Alpha mask Picture to render shadow. Affected by shadow opacity.
   Picture shadow_alpha_pict;
+  /// The value of _COMPTON_SHADOW attribute of the window. Below 0 for
+  /// none.
+  long attr_shadow;
 
   // Dim-related members
   /// Whether the window is to be dimmed.
@@ -391,6 +394,8 @@ typedef struct _options {
   wincond *shadow_blacklist;
   /// Whether bounding-shaped window should be ignored.
   Bool shadow_ignore_shaped;
+  /// Whether to respect _COMPTON_SHADOW.
+  Bool respect_attr_shadow;
 
   // Fading
   Bool wintype_fade[NUM_WINTYPES];
@@ -1063,9 +1068,18 @@ set_focused(Display *dpy, win *w, Bool focused) {
 static void
 determine_fade(Display *dpy, win *w);
 
+static void
+win_update_shape_raw(Display *dpy, win *w);
+
 static void
 win_update_shape(Display *dpy, win *w);
 
+static void
+win_update_attr_shadow_raw(Display *dpy, win *w);
+
+static void
+win_update_attr_shadow(Display *dpy, win *w);
+
 static void
 determine_shadow(Display *dpy, win *w);