Unify height maps

# Description:
A little something I'd completely forgotten about!
Currently we store two pointers to Signed and Unsigned 16 bit data for the heightmaps for Earth, Moon, etc. Then we convert them to doubles each time they're used. This is both wasteful and daft.

Instead I convert them on loading to double along with whatever other operations needed applying.

There's more that could be done with them, preconvert to cubemap perhaps, but this commit just tidies it up into a modify-once-on-load rather than modify-on-every-read.

Thought I'd submit it before I forget it exists again! :)

Andy