Drain holes are not allowed to be placed on its own inner surface. This was recently (0e3ebb3)
done by forcing invalidation of the slaposDrillHoles, which also invalidates and therefore
hides the drilled mesh. However, that also hides the hollowed mesh and it is not possible
to place holes inside the cavity.
This change does not dump the drilled mesh, but checks that no raycast hit ends up in a hole.
