stack: skip first frame for animations

stack: skip first frame for animations

Not the ideal solution for this problem, but in practice leads to
much better performance on lower end hardware.

Stack does a double draw on the first frame of its animation, of
both the old contents (into a cairo surface) and the new contents.
Homogeneous stacks only need to reallocate contents on the first
frame.

On lower powered hardware where our frames will be a good deal
slower than the refresh rate anyway, we can assure a smother
experience by waiting a frame to start tweening where frame duration
will be more consistent.