express: make WeakPointerTo cast operators explicit
express: make WeakPointerTo cast operators explicit

This prevents accidentally (and unsafely) decaying a WeakPointerTo to a regular pointer.

chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d