theme: Stop clipping titlebar buttons

There is no good reason to do so, besides a nice way to check whether
a particular button is enabled. However there are legitimate reasons
for overdrawing like box-shadows or outlines, so remove the clip.

src/ui/theme.c

@@ -770,10 +770,8 @@ meta_frame_layout_draw_with_style (MetaFrameLayout         *layout,
         gtk_style_context_set_state (style, state);
 
       cairo_save (cr);
-      gdk_cairo_rectangle (cr, &button_rect);
-      cairo_clip (cr);
 
-      if (gdk_cairo_get_clip_rectangle (cr, NULL))
+      if (button_rect.width > 0 && button_rect.height > 0)
         {
           cairo_surface_t *surface = NULL;
           const char *icon_name = NULL;