New frame flags for more complicated animations.

* FF_MIDDLESTARTCHANCE - has a 50% chance of starting the spr2 or FF_ANIMATE animation halfway in
* FF_SPR2ENDSTATE - if var1 == S_NULL, don't loop, just stop incrementing the frames. Otherwise, go to the state represented by var1.

The former is just something I did for fun, the latter is something that'll come in handy when porting in new-character-moves.