Element height

Finishes map_element_height (previously sub_662783) to include logic for slopes.

It includes subtle bugs from the original too! Elements which take up a whole tile aren't marked as such, and have (x,y) coordinates at the corner of the tile. As this routine seems to only be for locating objects rather than drawing them, it just means the camera will pan to a slightly wrong height, depending on the slope and current rotation.