gen_classic size limitation

[rakiru]

The gen_classic map gen basically works on a scale of 512x512 and sampling it at the given size. This causes problems when trying to generate maps larger than that (in this case, it breaks and you get the generic flat map). This should be changed to work with maps of any size, or at least increased to a much more feasible limit. It would probably work fine to just replace all the hardcoded sizes with ones based on the input sizes.

Also, the height is never scaled, which means small maps end up with really steep hills, and large ones less so. This should be addressed at some point, but isn't really in scope for this ticket.