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`.
* Add MakeRectangle helper to convert from SDL_Rect
* Add Rectangle::inset method for shrinking a rectangle
Turns out some of the other use cases I though this could apply to were actually doing something based on a fixed region
* Simplify initialisation of settings menu rects
Gleb Mazovetskiy
ephphatha
Juliano Leal Goncalves