cogl: Flush specific framebuffers

This resolves a couple of FIXMEs. The FIXME comments were right in
stating that not *all* journals needed flushing, only the one we
are trying to put on screen needs flushing.

However we can't eliminate all flushes because the winsys swap calls
that follow go directly into OpenGL which knows nothing about cogl
journalling. So the journal *must* be flushed before the swap, to give
OpenGL the correct state.

P.S. If this turns out to cause any bugs then the next best answer is
to just remove the FIXME comments. Because flushing is still the right
thing to do.

!1362
6 jobs for !1362 with flush-fixmes in 14 minutes and 17 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #905295
check-commit-log

00:00:14

 
  Build
passed #905296
build-mutter

00:09:15

passed #905298
build-without-native-backend-and-wayland

00:02:40

passed #905297
build-without-opengl-and-glx

00:06:17

 
  Test
passed #905300
can-build-gnome-shell

00:01:49

passed #905299
test-mutter

00:05:01