Fix sluggish performance in Safari by using timestamp from requestAnimationFrame to determine fade progress

[islandsvinur]

Hi Andre,

This fixes the sluggish performance in Safari. By using the timestamp that requestAnimationFrame provides to the callback, we can calculate the progress of the animation. Also tested in Edge, Chrome and Firefox on macOS, which all still behave properly.

I put the rebuild in a separate commit; didn't know if you want to rebuild it yourself for a release.

Fixes #315

[islandsvinur]

I just realised, for starting/target opacities < 1 the animation time will be shorter than fadeSpeed. With a fadeSpeed: 300 and a target or starting opacity of 0.5, the animation will now take 150 ms. Because the setting is called fade speed, it suggests a certain step size instead of the duration of the animation.

I will fix the code to keep the current behavior.

[Mtillmann]

@islandsvinur thanks for your dedication! Great work 🙏