Support split positions in animations

Animations for positions in Lottie can be either per (x, y) pair or you can have individual animations for x and y (for instance if you want different easing curves).

The VectorImage backend for Lottie currently does not support the individual x and y animations since this has not been implemented yet.

This also applies to animated vector properties (e.g. the scale property in transformations): The vector element may have individual easing curves.