If a pipeline has been flushed that disables depth writing and then we
try to clear the framebuffer with cogl_framebuffer_clear4f, passing
COGL_BUFFER_BIT_DEPTH then we need to make sure that depth writing is
re-enabled before issuing the glClear call. We also need to make sure
that when the next primitive is flushed that we re-check what state the
depth mask should be in.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 3cf497042897d1aa6918bc55b71a36ff67e560b9)
