[cogl_clip_push_window_rect] fix Cogl -> GL coordinate conversion
[cogl_clip_push_window_rect] fix Cogl -> GL coordinate conversion

Firstly this now uses the draw buffer height not the viewport height
when we need to perform a y = height - y conversion, since (as the
name suggests) we are dealing with window coordinates not viewport
coordinates.

Secondly this skips any conversion when the current draw buffer is an
offscreen draw buffer since offscreen rendering is always forced to be
upside down and in this case Cogl window coordinates == GL window
coordinates.

jigpu/clutter,djdeath/clutter,spatulasnout/clutter,collects/clutter,kerrickstaley/clutter-vala,Distrotech/clutter,nobled/clutter,dlespiau/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,ebassi/clutter,ebassi/clutter,Distrotech/clutter,jigpu/clutter,djdeath/clutter,ebassi/clutter,djdeath/clutter-android,ebassi/clutter,nobled/clutter,djdeath/clutter,djdeath/clutter,GNOME/clutter,rkudiyarov/clutter_osx,GNOME/clutter,GNOME/clutter,Distrotech/clutter,Distrotech/clutter,djdeath/clutter,djdeath/clutter-multithreaded,djdeath/clutter,jigpu/clutter,collects/clutter,GNOME/clutter,collects/clutter,ebassi/clutter,heysion/clutter-clone,jigpu/clutter,collects/clutter,collects/clutter,spatulasnout/clutter,dlespiau/clutter,Distrotech/clutter,jigpu/clutter,heysion/clutter-clone,rkudiyarov/clutter_osx,spatulasnout/clutter,spatulasnout/clutter,djdeath/clutter-android,rkudiyarov/clutter_osx,djdeath/clutter-android,dlespiau/clutter,nobled/clutter,spatulasnout/clutter,rkudiyarov/clutter_osx,jigpu/clutter,djdeath/clutter-android,heysion/clutter-clone,Distrotech/clutter,djdeath/clutter-multithreaded,heysion/clutter-clone,djdeath/clutter-android,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,GNOME/clutter,kerrickstaley/clutter-vala