PolyObject cleanup

Just a bunch of cleanup in the PolyObject code, in preparation for UDMF. I removed the commented-out "explicitly include line" code. It has never worked in SRB2, and at this point I'm not sure it would even compile anymore.

Setting the Upper Unpegged flag on the PolyObject parameter line now allows you to specify the translucency via the front X offset instead of the control sector's floor height.

srb2win-polyobject.exe