Blender Git Loki

Bug fix own collection:
- Recreate mipmaps if they're dirty when drawing background image
(this prevents "delayed" update of background image when you're paiting on it
in image editor).
- Mark mipmaps as dirty when inverting image channels.

TODO: there's memory leak caused by IMB_remakemipmap, but it'll be
anuther bugfix commit after discussion with other devs.