Fix #1025 and some P_PathTraverse bugs

This MR is a standalone version of the blockmap fixes from !2389:

• 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:
  • 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.