ClutterStageCogl: Clip in the right coordinate system

Our clip coordinates are relative to the stage, not model-view
transformed. cogl_framebuffer_push_rectangle_clip() was accidentally
used instead of cogl_framebuffer_push_scissor_clip() when porting
to the framebuffer clip API.

https://bugzilla.gnome.org/show_bug.cgi?id=719900