Add presized arena allocator.
Load all mip levels from an RGT file rather than just the largest.
Remove FOLDIMAG loading placeholder.

While previous Relic games have done, CoH2 doesn't support FOLDIMAG.
Dont try to load unsupported FOLDDXTC versions.

CoH2 does something for FOLDDXTCv1 and FOLDDXTCv3, whereas the code here only handles v3 at the moment.