Blender Git Loki

Gooseberry request:

Show World will now influence if world is rendered in opengl rendering.

This is a little undefined according to blender history, since sky used
to always be drawn when offscreen rendering, as if "Only Render" was
ticked. Since if we don't draw sky in that case there's no valid color
really (and using theme colors is not so nice) we just draw transparent
background.