Added RF_ABSOLUTEOFFSETS and RF_FLIPOFFSETS

- Added render flags (see r_defs.h)
- Implemented floor splats
- Drop shadow sprite rendering through render flags

Replace all instances of SF_CANBREAKWALLS and PF_CANBREAKFLOORS with SF_CANBUSTWALLS and PF_CANBUSTFLOORS, respectively.

The wiki even states its existence!

* added POF_ flags to INT_CONST in dehacked.c

someone entertained the idea of maybe removing it, so might as well put this merge request up to see if people want it

WIP because I don't know if this wording is right for the error.