UDMF: The whole thing

This branch finishes the UDMF implementation - for now! (There's still going to be new features in the future, but this branch should be fully functional.) Based on !1696 (merged), so merge that one first.

Here's what's new in this branch:

• Some code cleanup.
• Lower Unpegged no longer affects the pegging of the midtexture in UDMF maps. That's done exclusively via the Peg Midtexture flag now.
• In UDMF maps, FOF texture skewing is now enabled by checking the Slope Skew flag on the control linedef instead of Upper Unpegged.
• Implemented a faster TEXTMAP parser. UDMF maps now load almost as fast as binary maps.
• Removed the writethings command because the internal binary-to-UDMF conversion breaks it. Also disabled NiGHTS Objectplace because it's useless without writethings.
• If you run SRB2 with the -writetextmap command line parameter, then every time the game loads a map in binary format, it will save the converted UDMF map as a TEXTMAP lump into the SRB2 folder. There's a few things the converter can't handle, but it will warn you about those in the console.

srb2win-udmf.exe