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