Shared skin resources

[Zemanzo]

Description
Currently, every skin has their own set of resources. This makes management easy, but has a significant impact on the client, as they might have to download identical files multiple times.
For example, all poolball skins use the same normal map. It would be pointless to download it separately for every variation.

Minimum Acceptance Criteria

• Skins resources can be shared by defining so in the meta.json file.
• A default vertex shader should be present for shader based skins, as it is generally identical. A custom one should still be supported.

[Qaomen]

I don't think not having shared skin resources is currently an issue. Normal maps are generally unique to the marble skin, with the exception to the poolball skins. Adding to that, the normal maps of the poolball skins are visually so insignificant that I think the better option is to simply remove these normal maps entirely.

The default vertex shader could be a useful feature, though. We can opt to define a hard-coded vertex shader whenever a skin's vertex.glsl doesn't exist.