animate: Fix crash when unloading plugin during animation

[soreau]

Fixes #670.

[ammen99]

This change doesn't make much sense. We explicitly want to clean up the running animations of views not any output when unloading the plugin (we need to clean up everything in this case anyway), and your proposed change will just ignore views not on any output - but if they have any pending animations, we should remove them for sure.

I suspect the actual bug is somewhere else, we probably destroy the views when removing the animation from them. Likely it would help to get the views from core, make them into shared pointers (so that they are alive as long as the function runs), remove all transformers (the shared_ptr will keep them alive) and then they will be freed once the function has finished running.

[ammen99]

Superseded by #2142