Revert "framebuffer: Don't mark the clip dirty at mid scene"

This patch doesn't look right because now nothing will ever set
clear_clip_dirty = TRUE. Presumably that would mean that if a
rectangle is drawn and then the journal is flushed before the
framebuffer is read, then it would think it could return the clear
color even though it shouldn't.

Perhaps a better approach would be to make a second version of
_cogl_framebuffer_mark_mid_scene that doesn't set the clear_clip_dirty
flag and call that from the journal instead. As this patch was pushed
without review and without first going into the master branch I think
it makes sense to just revert it and apply a new version to master.

This reverts commit 3eb63f67a34420b30ef812a1610aa9e9a80a93a3.