Mirror modifier clipping not accounted for in RF4 grab

[jlampel]

When using PolyPen with the mirror modifier, the vertices can move past the axis of symmetry when clipping is enabled. They should stop at the symmetry line, or 'stick' if they are already at the symmetry line, just like with the regular move tool. I'm assuming grab is the only custom transformation we're using at the moment, since rotate and scale seem to work with clipping already.