Galaxy image refactor

I found the issue #1590 (`New Color Galaxy Map Available`) with some much prettier galaxy images by @Baobobafet

Which led to me trying to load a separate colour image and then to find that the way we used the existing `galaxy.bmp` was absolutely horrifically slow. 

So I rewrote it slightly and it's now ~10x faster:
> Galaxy test took: 204.851104 milliseconds, totalVal (73381569)
> Galaxy test took: 203.604323 milliseconds, totalVal (73381569)

Versus the newer version:
> Galaxy test took: 25.115136 milliseconds, totalVal (73381569)
> Galaxy test took: 24.076669 milliseconds, totalVal (73381569)

Andy