Detect and handle tile misalignment

[ohrrpgce-bugbot]

[bz#810]

If you change the speed of a hero or NPC mid-step, you can possibly get stuck because of the hero (or NPC) getting misaligned with the tiles.

It shouldn't be too hard (with a little code cleanup) to detect tile misalignment, and handle it.

possible handlers:

* No handling, the current default misalignment causes stickage until a script fixes it manually. (should "wait for hero" throw an error when misalignment stickage is detected?)

* Tweak position to allow the move to finish. When stickage is detected, check to see what the walker's current speed is, and what their distance to the next tile is, and nudge them forward the correct number of pixels to make sure they are realigned.

From: @bob-the-hamster
Reported version: 20110506 Zenzizenzic

[rversteegen]

Duplicate of #660.

In fact, the specific problem mentioned here, NPCs getting misaligned when speed is changed mid-step, was fixed in Fufluns.