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 3eb63f67a3.

cogl/cogl-framebuffer.c

@@ -235,6 +235,7 @@ _cogl_framebuffer_clear_without_flush4f (CoglFramebuffer *framebuffer,
 void
 _cogl_framebuffer_mark_mid_scene (CoglFramebuffer *framebuffer)
 {
+  framebuffer->clear_clip_dirty = TRUE;
   framebuffer->mid_scene = TRUE;
 }