Bug fix: Fading blocks in rare circumstances

- In very rare circumstances, poll() to the X connection returns 1 but
  no events are read out, causing XNextEvent() in the main loop to wait
  infinitely until another event comes, typically affecting fading
  process only, causing fading to appear somehow stopped. This commit
  adds a (possible) fix.

- Listen to Expose events of the X Composite overlay window if we are
  painting to it, to avoid making some parts of the screen blank when
  switching out of X screen in --paint-on-overlay mode.

- Drop "fade_fin" member of struct _win, because it's pretty useless.

- Drop unused "root" parameter of expose_root(), move get_time_ms() to
  compton.h, etc.

src/compton.h

@@ -235,8 +235,6 @@ typedef struct _win {
   Bool fade;
   /// Callback to be called after fading completed.
   void (*fade_callback) (Display *dpy, struct _win *w);
-  /// Whether fading is finished.
-  Bool fade_fin;
 
   // Frame-opacity-related members
   /// Current window frame opacity. Affected by window opacity.
@@ -696,8 +694,20 @@ free_damage(Display *dpy, Damage *p) {
   }
 }
 
+/**
+ * Get current system clock in milliseconds.
+ *
+ * The return type must be unsigned long because so many milliseconds have
+ * passed since the epoch.
+ */
 static unsigned long
-get_time_ms(void);
+get_time_ms(void) {
+  struct timeval tv;
+
+  gettimeofday(&tv, NULL);
+
+  return tv.tv_sec * 1000 + tv.tv_usec / 1000;
+}
 
 static int
 fade_timeout(void);
@@ -714,8 +724,7 @@ set_fade_callback(Display *dpy, win *w,
  */
 static inline void
 check_fade_fin(Display *dpy, win *w) {
-  if (w->fade_fin) {
-    w->fade_fin = False;
+  if (w->fade_callback && w->opacity == w->opacity_tgt) {
     // Must be the last line as the callback could destroy w!
     set_fade_callback(dpy, w, NULL, True);
   }
@@ -966,7 +975,7 @@ static int
 error(Display *dpy, XErrorEvent *ev);
 
 static void
-expose_root(Display *dpy, Window root, XRectangle *rects, int nrects);
+expose_root(Display *dpy, XRectangle *rects, int nrects);
 
 static Bool
 wid_get_text_prop(Display *dpy, Window wid, Atom prop,