window: Use guint8 for opacity internally

Except while reading _NET_WM_WINDOW_OPACITY, opacity is between 0 and 255. With
guint8, we'll get compiler warnings if arbitrary int values are passed.

https://bugzilla.gnome.org/show_bug.cgi?id=727874

src/core/window.c

@@ -9129,7 +9129,7 @@ meta_window_set_transient_for (MetaWindow *window,
 
 void
 meta_window_set_opacity (MetaWindow *window,
-                         guint       opacity)
+                         guint8      opacity)
 {
   window->opacity = opacity;