[Bf-codereview] Alpha premul pipeline cleanup (issue 7018048)

[brechtvanlommel at gmail.com]

With the extra premultiplication in ibuf_get_color texture filtering
works mostly correct, however mipmapping (IMB_makemipmap) is still
wrong. There it should also premul/unpremul as it scales down the image,
to get correct interpolation.

A compatibility issue here exists also when users have set up an image
as a texture but don't want to use its alpha channel (Use alpha
disabled). In this case premultiplying in texture filtering will make
some areas black. Maybe this is acceptable, or maybe the texture
filtering should automatically skip the premul in that case? For mipmaps
it's a bit weak if the image is used by multiple textures with different
settings, but usually would go ok to skip it.

In compositing and the sequencer, color correction type nodes should
have an option to unpremul the color. Not necessarily something that
should be done as part of this patch, just making a note.

https://codereview.appspot.com/7018048/