Implemented delayed partial rect update for image buffers

Used by image painting mode, so every brush step would mark area
if affected on, but actual color space conversion would happen
later when actually displaying image.

Implemented as a rcti stored in ImBuf which is getting merged with
partial rect passed to IMB_partial_display_buffer_update_delayed.

This makes painting as fast as it currently possible and finally
solves #33935: Texture painting slow down with mouse, but not with tablet

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54463 954f8c5b-7b00-dc11-b283-0030488c597c