No more dropping from doors!

Fixes #1755 - issue's only a year or so old. Adds the maths so player spawns in correct place on entering from the command line or the overworld and fixes the positions of a lot of doors so they line up with the floor, solving the issues with in-game dropping.

In 2D levels we want the bottom of the door to line up exactly with the top of the block below. Any higher and the player drops. Any lower and they fall through the floor and die (this happened a lot in Greendale - should have done more testing).

In 2.5D levels we want the bottom of the door to be overlapping the edge of the floorspace just slightly. Any higher and the player drops.

This pull also fixes minor collision issues in several levels - chairs were too high in hallway and there were issues with slopes in new-abedtown.