[cogl-draw-buffer] fix Cogl -> GL viewport coord conversion
[cogl-draw-buffer] fix Cogl -> GL viewport coord conversion

Before we call glViewport we need to convert Cogl viewport coordinates
(where the origin is defined to be top left) to OpenGL coordinates
(where the origin is defined to be bottom left)

We weren't considering that offscreen rendering is always upside down
and in this case Cogl coordinates == OpenGL coordinates.

djdeath/cogl,djdeath/cogl-android,spatulasnout/cogl,djdeath/cogl,Distrotech/cogl,spatulasnout/cogl,djdeath/cogl,Distrotech/cogl,gcampax/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,Distrotech/cogl,djdeath/cogl-android,collects/cogl,collects/cogl,Distrotech/cogl,gcampax/cogl