[read-pixels] don't flip data when reading from offscreen draw buffers
[read-pixels] don't flip data when reading from offscreen draw buffers

Since we do all offscreen rendering upside down (so that we can have the
origin for texture coordinates be the top left of textures for the cases
where offscreen draw buffers are bound to textures) we don't need to flip
data read back from an offscreen framebuffer before we we return it to the
user.

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