Add support for remaining features

[gkjohnson]

• Skinned Meshes
• Understand replaceable textures tables -- See description for skinfamilies and tables in the MDL Wiki
• Understand bone tables
• Understand external models
• Add an eyeball shader for eyes (parameters are defined in the material)

For Animations

• Need model that comes with animations -- most seem to just be rigged
• Understand animation blocks
• Understand animation controllers
• Understand animation sequences

Low Priority

• Object attachment points mstudioattachment_t
• Understand flex descriptions, more -- they're basically morph targets
• Understand IK Chains
• Understand mouths
• Understand pose param descriptions
• Understand surface prop
• Understand ik lock
• Physics mass
• Understand LOD updates
• Hitboxes