Add support of Source engine v44 mdl format (.mdl, .vvd, .vtx)

[Duude92]

This PR adds Source engine v44 models support.
Provide multiple structs for reading different Source engine model-related formats.
Although this PR provides structs per files - it is not fully tested (e.g. I haven't looked into animation sequences and how do they work)
Screenshot with some tested models:

Sorry, I haven't bother to make unit-tests.
Also, probably later would test for else mdl version, and/or do fixes
Also, quick reference to code:
Since model data is seprated between multiple files - I've provided some properties/methods to 'easily' retrieve data from MdlFile object, e.g.:

• BodypartCount => Header.bodypart_count
• LodCount => VtxFile.Header.numLODs
• GetModelCount(int bodypart) => Bodyparts[bodypart].Header.nummodels
• GetMeshCount(int bodypart, int model) => Bodyparts[bodypart].Models[model].Data.nummeshes
• MeshVertex[] GetVertices() -> this precomputes vertex data with vvd fixups
• GetIndices(int meshIndex = 0, int lodIndex = 0, int modelIndex = 0, int bodyPart = 0) -> returns precomputed array of indices per mesh
• GetMaterialIndex(int meshIndex = 0, int modelIndex = 0, int bodyPart = 0) => Bodyparts[bodyPart].Models[modelIndex].Meshes[meshIndex].Data.material -> material is bound to mesh

[Duude92]

Hey @LogicAndTrick, can you review PR?

[LogicAndTrick]

Thanks for the implementation, but to be honest, I cannot review this - there's 1400 lines of code and no tests. I think its worth having this branch available to compare with another implementation though, since based on your screenshot it looks like it works well.

Doing a quick scan through the code, its doing things that I specifically avoid in other parts of the lib - unmanaged memory/memory pinning, PtrToStructure, code copied directly from the Source SDK (license issues).

So unfortunately I won't be merging this, sorry :(

[Duude92]

I've used Marshaling just because it's faster to implement, so, maybe, someday, I may rewrite it for parser.
About licensing, I thought format specifications are not copyrightable, even quick search leads to answers that specifications (not implementations) cannot be licensed. I think there shouldn't be any legal issues.
Anyway, thanks for the reply, no - means no, you can close this if you wish.