Implement beat-synchronized animation in skip overlay

[nekupaw]

I think the new beat-synced animation in the break overlay is really cool, so I implemented pretty much the same approach in the skip overlay.
I also rounded the corners by replacing the box with a circle.

Let me know what you think.

skipOverlay_beat-synced-animation.mp4

[bdach]

Seems fine? The biggest qualm I'd have is that there's no real guarantee that any beatmap intros or outros are properly timed to the beat, because people just might not have bothered. Not sure how frequent that is.

@nekupaw I fixed your commit message. "publish" is not a good commit message. Please write better ones if you wish to continue contributing.

@peppy Requesting second check on whether we want this at all and whether the visuals look ok.

[peppy]

Not sold on the rounded corners against the hard edge. But this whole overlay will likely be refined in the future™.