Without doing a stroke "manually", colorspace initialization on
UnifiedPaintSettings was never done.
This lead to wrong colors in the past, since modern CS code with OIIO
this also crashes.
Colorspace was added into UnifiedPaintSettings in 6f153046e0 to avoid
doing this when sampling the brush texture images [which is good], but
the initalization was done in `paint_brush_update`.
We dont need to have this called for every step (it was skipped anyways
after one run), so it seems we can move this out of `paint_brush_update`
and put this into `paint_stroke_new`.
Thx @RevDr for initial findings.
Pull Request: https://projects.blender.org/blender/blender/pulls/117756
9aa20a0979