Spacebox

# Credit:
I take no credit for this as it was originally written by @Salwan for Paragon. I'm just back porting it :)

# Description:
Add support for a skybox (spacebox?) displayed in the background to augment the stars and replace the Milky Way.

# Randomisation:
Currently there is just a single default cubemap however others can be added as a mod, or if we decide to use them for Pioneer, by putting files in the folder `"/textures/cube/"` and naming them `ub0.dds`, `ub1.dds` ... `ub9999.dds`, etc. 

You will need a program to make a cubemap that is compressed to DDS cubemap format and DXT1.

When you have a selection of them the game will pick a different one for each star system you jump too. It will always pick the same one for each system so you won't get a different backdrop everytime you visit the Earth.

For this commit, there is just a single default map, so the same one for every system.

Andy