Adds a custom sized type for the world tile rectagle.
This allows us to better express intent.
It also allows us to make certain globals smaller, e.g. `THEME_LOC`.
We want to be able to use unpacked MPQs on low-end platforms
(PS2/rg99/etc).
This is tricky on case-sensitive filesystems. Avoids case issues by
lowercasing all paths in the code (then we'll just need lowercased
listfiles).
Anders Jenbo