Global heightmap refactor

[astrsh]

Rework all terrain to use a global height map, rather than determining general height via biome distribution. This will make height variation look significantly better as terrain won't need to be interpolated so much. Biome specific detailing can be done by different EQs that utilize the heightmap in different ways.

Here are some examples of an early implementation

[astrsh]

#123 should be significantly easier with this change