For composting sky behind alpha, a gamma corrected alpha_under was used,
but Blender didnt make the gamma tables unless OSA was set.
Old bug!
Changed; while rendering with "Backbuf" that doesn't exist, blender still
renders, but without backfbuf now. It used to stop render and return, with
only a print in console... confusing.
Ton Roosendaal
ac9f9298fd