Support for linear() easing function

[mattgperry]

Currently, when a WAAPI animation is defined with an easing function:

animate(
  element,
  { opacity: 0 },
  { ease: (p) => p ** p }
) 

Or an easing name not supported by WAAPI:

animate(
  element,
  { opacity: 0 },
  { ease: "anticipate" }
) 

Then Motion will pre-generate keyframes for this animation and run it with linear easing.

This can have poor performance characteristics: https://issues.chromium.org/issues/41491098

Likewise for complex values like clip-path and filter this can have the overhead of number -> string conversion.

This PR adds support for the linear() easing function. So when an easing isn't supported by WAAPI, we generate a linear() easing function instead of pre-generating keyframes.

[adamseckel]

If you made a type guard like:

function isUnsupportedEase(key: string): key is keyof typeof unsupportedEasintFunctions {
  return key in unsupportedEasingFunctions
}

You could avoid the casts.

[mattgperry]

ty will implement

[adamseckel]

Why 10ms btw?

[mattgperry]

just somewhere between 8 (120fps) and 16 (60fps) that looks quite good without being too accurate (as there's a resolution/performance tradeoff)