Expose P_PathTraverse to Lua

Exposes P_PathTraverse to Lua, allowing modders to search objects and linedefs touching a line within specified coordinates. Also exposes PT_ADDLINES, PT_ADDTHINGS, and PT_EARLYOUT to pass in as flags for P_PathTraverse.

This MR also fixes some blockmap bugs:

• Fixes #1025 (closed) by adding a new argument to P_BlockThingsIterator that takes an mobj_t and checks for it, instead of just checking for tmthing.
• Fixes a few bugs with P_PathTraverse itself:
  • Ports over a couple of fixes regarding path traversal from ZDoom (traversing through blockmap corners, etc.).
  • Fixes incorrect calculation(s) of partial (now partialx and partialy), xstep, ystep, xintercept, and yintercept.
  • Path traversals can now go beyond a map's blockmap limit, granted they stay within the hardcoded limit of 65536x65536.

Example Lua: P_PathTraverseTesting.lua

Special thanks to the following users for helping me out with this MR:

• Golden (This MR was based off of the p-path-traverse-lua-exposure branch, which Golden worked on)
• Monster Iestyn (For helping me figure out how to pass in a Lua function as a parameter)
• AlamTaz (For helping me figure out why my branch was failing to build)
• Anyone else who helped out (I hope I didn't forget anyone...)