Transitioning to the `next` animation frame preserved the priority,
which was a problem for enemies with a next: Hit=Idle or similar
transition, as that would give an infinite Idle loop with high
priority.
This change forces a clear of the animation priority when such a
transition occurs. This relies on the flawed linked list system, which
means that it will not happen if the new animation is just after the
current one in the source file. This is a fundamental issue due to
mixing all frames as single pointers in the anim_t structure. This will
be fixed in engine code eventually.
Lephenixnoir