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
7fcc1ac48b