Fix virtual POF crashes

[BMagnu]

There are two memory management issues that cause crashes with virtual POFs:

1. Loading a virtual POF that is already loaded will error out, because virtual POFs have their filename set incorrectly to the base source POF, causing cache lookup to fail and to report inconsistent model numbers.
2. Using the same submodel multiple times as a source in the same POF causes shared data in the polymodel struct. When the polymodel deallocates, this will cause multiple frees (once for each submodel) on the shared data, causing a crash. Fixing this can either be done by deep-copying the entire submodel data (which is waaaay too much hardcoding) or by managing the polymodel data properly with smart pointers. Hence this PR refactors all dynamically allocated memory for polymodels to use shared_ptrs instead.