This refactors FontManager to improve it in several ways:
1. Change all the dynamic memory management to use `new`, rather than mixing `new` and `vm_malloc`
2. Change the current font to use an index rather than a pointer
3. Take advantage of more C++11 features to avoid redundant lookups
4. Use case-insensitive name comparison
5. Avoid overflowing the `fnt->filename` field
6. Use `Warning` rather than `Error` in a non-critical error situation
Goober5000
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