Improvement: Change window type detection

- Let window type detection start with the client window if there's one,
  in hope to enhance performance.

- Change get_wintype_prop() to fetch the property only once.

- Default to WINTYPE_UNKNOWN instead of WINTYPE_NORMAL if
  _NET_WM_WINDOW_TYPE is missing.

- Fix a mistake in calc_opacity().

- Add some items to .gitignore.

- Fix a typo in usage().

src/compton.h

@@ -588,7 +588,7 @@ static wintype
 get_wintype_prop(Display * dpy, Window w);
 
 static wintype
-determine_wintype(Display *dpy, Window w, Window top);
+determine_wintype(Display *dpy, Window w);
 
 static void
 map_win(Display *dpy, Window id,