Blender Git Loki

Reverted the change of auto-turning on premul for images from last
commit. I still think this is a good idea, but needs more discussion.

Basically, the way the premul option worked before is it actually
changed the image data to be premul (for each pixel it multipled r,g,b by a) when
loading an image. So if a user wanted his image to be key, yet
still work in the renderer (which expects premul) he'd be stuck.
Also, it was kindof confusing how if you painted something in the image,
then saved it with premul on, when next you loaded the image it'd have
changed (especially since the image editor painting seems to paint in
key, not premul).