Blender Git Loki

Fix T93130: Frame Selected with selected paint mask does not work

This broke with {rB20fac2eca723} (which landed in 2.63), so long
standing bug.

Convention for paint modes is:
- when no paint mask is active, `Frame Selected` will focus the last
stroke
- when paint mask is active, `Frame Selected` will focus the selected
mask faces

To check the right vert coords we have to offset with `mp->loopstart`.

Maniphest Tasks: T93130

Differential Revision: https://developer.blender.org/D13247