More floating point movement bugfixes

(A lot of this is resolving merge conflicts with master. New commits start at e53a62e)

With these changes, floating point movement is "100% done" as far as I can tell. A quick summary of the important changes:

* Add a half tile adjustment to `map_to_screen()`. I don't entirely understand why what I did works, but it makes rendering line up when comparing directly to the master build
* Fix some things not being positioned on the center of tiles (e.g. teleport destinations, loot, some enemies, etc)
* There were quite a few typing bugs with points where we were still using Point instead of FPoint

Hopefully, I didn't miss anything. If you want to test, my `fp_movement` branch on flare-game is up-to-date  https://github.com/dorkster/flare-game/tree/fp_movement