Possible overflow when loading models; overlap with fake model handles.

This adds MAX_SUBMODELS, initially set to 512. This avoids breaking netcode
compat.

The fake handles for box and capsule are adjusted so as to be outside the
submodel handle range.

Closes: #268, #269, #270

Ref. https://github.com/zturtleman/spearmint/commit/d1c612ee3623a6b25050f0c913a713184f7cf859