Don't use interpolation code if the frame lasts instantaneously or infinitely

Also removed the + 1 from newtime, since there was never really any need for it. It just offset the interpolation so it went like (1 -> 2] instead of [1 -> 2), so you never saw the base appearance for each frame except at the end of any frames interpolating to it

Changed DrawMD2Ex's duration/tics type to INT32 so -1 comparisons work, probably want to change the signs elsewhere too but this is fine for now