[cogl_read_pixels] use buffer not viewport height to calculate y offset
[cogl_read_pixels] use buffer not viewport height to calculate y offset

glReadPixel takes window coordinates not viewport coordinates so we
shouldn't have been assuming that the viewport height == window height.

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