- Port based on Baystation's implementation and customized for our code and usages.
- Switched space icon_state permutation to a "dust" overlay that is permutated the same way. As an overlay, the dust can be *above* the parallax planes. Space turfs are now white to allow skybox to BLEND_MULTIPLY onto them.
- Added skybox hud effect that replaces background space turfs with an image moved to create a parallax effect.
- Added skybox subsystem to generate and manage skybox parallax images, and lots of images for it!
