libweston: Return NULL from creation of animations that have finished

Animations can be destroyed before their creation functions return. This draft MR proposes a solution, and implements it for one of the animation types.

The downside is we have no way to differentiate between failing to create an animation and successfully "instantly" completing an animation. This could be fixed by throwing an additional bool in the mix, but I'd like opinions on whether that complexity is necessary before I move on to fixing up the rest of the animators.