Do a bunch of stuff to the MD2/3 sprite2 support to get it back to feature parity with before, without going back to being hacky as fuck.

* Store the number of frames per sprite2 run in the spr2frames struct.
* Reintroduce P_GetModelSprite2, to allow for the sprite2 defaulting system to be used to full advantage even in GL.
* Instead of splitting the SUPER and normal SPR2 stuff within the same cell of the struct, have them exist in different cells just like in the "normal" sprite2 structs.
* Allow for just providing spr2 frames in order without specifying which "normal" sprite2 frame it's supposed to replace.

Also:
* Fix FF_VERTICALFLIP-ignoring regression.
* Fix whitespace adjustment in win_dll.c
* Remove bracket in P_GetSkinSprite2 because I realised it was extraneous while making sure P_GetModelSprite2 worked with it.