Allow wandering across map edges.

Introduce 'ModDict' type for RealMap - wraps coordinates to world size.
Tile the 'world' surface at edges and corners.

Some glitches - minimap view rectangle wanders out of bounds,
view scrolls weirdly when wrapping from one side to the other.
Fix minimap rect (HACKY!)

Loadsa code duplication.
Create one bit of code that draws
these rects and the main window tiles
as required and tidily.
Fix weird jumping scrolling.
Replace horrendous code with sanity.
Make scrollPos a tuple again, rather than a list.
Remove hacks put in for ease of testing.
Modify map to wrap nicely and allow walking in circles.
Without a corrupted image this time.