Bug: Mask polymorphs do not handle light emitting monsters

[elunna]

apply_mask directly calls polymon, usually the player poly behavior is handled in polyself - and the light source of a new poly form is handled in polyself (but it oursources the actual poly to polymon().

In the case of masks we don't get the light handling - resulting in a "del_light_source: not found type=1, id=55c64cd2f9b0" when the mask poly wears off.

This was originally found here: elunna/hackem#19
For my fix see this commit: elunna/hackem@bb72213

I just copied the light source handling to apply_mask.