There's a bit of flickering when unpausing where the node has the original
state, but I'm not totally sure where it comes from, posibly from
PropertyAnimation returning None due to no styles changing?
There are a few shortcomings, for example:
* We don't do the same as other browsers when some properties are not specified
in some of the keyframes, though this is easy to work out (though a bit more
expensive in the sense that we should apply all the previous keyframes' style
instead of just the previous and the next.
* To trigger the initial animation, a restyle is necessary. Should be easy to
do an initial restyle off-hand or something like that, but for now this is
worked-around adding a :hover rule to the node.
Also, the animation is resetted when the node is hovered. That's a bug, but
is probably not so difficult to test.
* A few things, mainly animation-direction, are not supported yet, but
shouldn't be that hard to support.
Still a lot of work to do, but I think this approach might be ok.
There's some maths I've done wrong, but it DOES animate some things, though
they're only triggered past the first restyle, and we probably have some
duplications where the animations arrive to layout.
Anyway, got to go.
So this actually allows some more animations to be triggered. The bad part of
this is that they're actually triggered always when the style is recalculated,
so we're going to have at least some more state into the node, and the
constellation, which would have to keep track of the animations states.
Use our copy of RefCell for style data
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This allows removing `#![feature(as_unsafe_cell)]` in geckolib and make progress towards #11815.
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