Remove polymost stuff

- Maybe it will be back someday, but it's been essentially dead for nearly
  10 years, so don't hold your breath.
Consolidate wall texturing vars into two structs

- All transformation and clipping values go into FWallCoords.
- All texture mapping values go into FWallTMapVals.
Pass WallC as a parameter to (O)WallMost instead of referencing it globally
Pass x1 and x2 to Prep(L)Wall as params
Consolidate some common code for texture mapping setup

- R_AddLine() and R_RenderDecal() had nearly identical code for setting up
  texture mapping. These have now been spun off into methods of
  FWallCoords and FWallTmapVals.