This patch just adds a call to _cogl_framebuffer_flush_state to ensure
the correct framebuffer is bound before discarding its buffers.
Previously it would presumably just discard the buffers of whatever
framebuffer happened to be used last.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 37c390a5b33d4f65ff6c834e9be2f8de716635ee)