When the brush size is bigger than the entire mesh, fdata.tot_co can be
0, so the pose origin will default to (0,0,0), which does not make much
sense. After this patch, the pose origin will be set to the farthest
vertex from the pose origin, which at least should be in the surface of
the mesh and in most cases in the direction the pose brush was already
detecting the origin.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D7773
9aea7dc7c6