Some lines were being drawn at non-integral positions.
The x and y coordinates are now floored before use.

Entities can now be rotated. This does not affect
their height. For now, they can only be rotated by
45 degrees at a time. I will work out something nicer
later on.

Permalinks now just stick the base64 string into the
fragment portion of the URL, rather than using a
query parameter. This should reduce spam in the server
logs, and also keep scenes more private.