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

source/blender/editors/mesh/editface.c

@@ -369,7 +369,7 @@ bool paintface_minmax(Object *ob, float r_min[3], float r_max[3])
       continue;
     }
 
-    ml = me->mloop + mp->totloop;
+    ml = me->mloop + mp->loopstart;
     for (b = 0; b < mp->totloop; b++, ml++) {
       mul_v3_m3v3(vec, bmat, mvert[ml->v].co);
       add_v3_v3v3(vec, vec, ob->obmat[3]);