DDS for all models

This replaces all of the PNG files with DDS files.

PNGs can of course still be loaded but ideally we would have ONLY DDS files in the distributed version of the game. These load faster and guarantee quality across all platforms without relying on that platform/drivers ability to compress them.

Also they compress better so the download would itself be slightly smaller :)

Currently they are all DXT5 (I think) which is not necessary, many could be DXT1 due to having no alpha channel.

The original PNG files [should go into](https://github.com/pioneerspacesim/pioneer-assets/pull/10) `pioneer-assets` along with geometry etc.