Load xac and xsm with cpp

[Nemrav]

New PR - needs code review.

XSM animation file loading:

Can now read 0xC8 chunk style animations (needed for zulu_walking.xsm)
Uses a hack of deleting frame 0 from all animation tracks to work. This replicates the behaviour of the gdscript version, but obviously is not ideal.
Cached in ModelSingleton
Used for all animation loading

XAC file loading:

Used for static (ie. no 'unit' script for animation) meshes only currently.
Textures are loading, but lighting is off
Bone weights / skinning chunks are not working and needs more work before this can fully replace the gdscript function versions.
Cached in ModelSingleton