Fix #1025 and some P_PathTraverse bugs

MIDIManrequested to merge
MIDIMan/SRB2:fix-blockmap-bugs into next

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

  • 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.

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