Add Half-Life 2 mounting support

[Asphaltian]

Adds mounting support for Half-Life 2 assets (models, materials, textures, sounds), with additional assets from the Episodes and Lost Coast

What's currently included

• VPK parsing and asset loading
• KeyValue parser
• MDL/VVD/VTX/PHY/ANI model loading and conversion
• VTF texture loading
• Shader recreations: VertexLitGeneric, LightmappedGeneric, UnlitGeneric, Eyes, Teeth
• WAV/MP3 loading

Known issues

More can be added down the road.

• Missing shader recreations for further materials
• Some models have incorrectly oriented meshes
• Models don't have flex animations, bodygroups, skin families, hitboxes, etc.
• Certain shader parameters use defaults despite being set (e.g. eyes and teeth materials)