Revert "Anchor unit overlays to the topleft corner of the sprite."

This reverts commit 8f49cbad5508d761c1139c65e064c31ea3c79a00.

This commit breaks the game when you zoom in, since I'm trying to
make changes to how that works it is inconvenient to have to deal
with it right now. This bug was assigned to lipk more than a month
ago when he first introduced it.

Conflicts:
	changelog
	src/unit_drawer.cpp
as a test, use xBRZ when zooming images

This also sets the max zoom to 288 so you can reach 3x and 4x if
desired.