Snowycoast Model & Texture Assets B5

Reduced some of the artifacting on the skybox.

Unfortunately the skybox just needs to be reauthored due to rescaling issues, so it's still got a lot of banding.

* Added new metaldoor sign textures (corresponding to snowycoast B6)
* Added fixed crate cinematic prop (corresponding to snowycoast's official update last month)
* Added larger versions of metal_bolt.mdl, metal_bolt_cluster.mdl, and metal_bolt_full.mdl

I should probably make a prop zoo for this stuff at some point.