Instead of extracting item sheets one at a time, it's now done on
multiple threads. Loading gear is now faster since reused files are
cached, switching between races is still wasteful, but it's a good
enough improvement for now.
This is a major code overhaul for mdlviewer, which will make it easier
to extend and modify in the future (trust me, the old code was garbage).
The different views are now split up (SingleGearView, FullModelViewer,
and MDLPart) which makes the functionality easier to handle, and less
error-prone.
Right now bone debugging is disabled (not that it worked that well
anyway) but will be brought back in a future commit.
