framebuffer: NOP _set_color_mask if mask isn't changing

This makes cogl_framebuffer_set_color_mask immediately bail out if the
given mask equals the framebuffer's current mask, since the cost of
flushing the journal and flushing the gl state will hugely outweigh the
cost of the check.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 925174d99df7f1f4b11098e748bcc23eaa396a21)

cogl/cogl-framebuffer.c

@@ -1248,6 +1248,9 @@ void
 cogl_framebuffer_set_color_mask (CoglFramebuffer *framebuffer,
                                  CoglColorMask color_mask)
 {
+  if (framebuffer->color_mask == color_mask)
+    return;
+
   /* XXX: Currently color mask changes don't go through the journal */
   _cogl_framebuffer_flush_journal (framebuffer);