* Added support for sprite2s to MD2s!

    - Name each frame either SPR2_**** or SUPER**** (where **** is the 4-character name)
    - If the name is 3 characters, '.' is accepted as a substitute for the '_', but a space/absent isn't (for tool-related reasons).
    - Adds a big sprite2 index array to all models, even non-player ones. Sorry!
* Made MD2 frame interpoleration only work across the same spriteset (and sprite2set).
* Made MD2 frame interpoleration happen when there's less than a quarter of a second between frames, as opposed to the hardcoded specific animation disabling.
* Fixed sprite2-related typo in dehacked.c.