Spread offset

Spread missing offset properties to keyframes with interpolated values.

This is a similar implementation to the programatical offset interpolatgion used in the native ELement.prototype.animate() method. Useing keyframes parsed and not parsed by spread offset in an animation will result in the same results. This is just usefull for customt tween animation engines or if youd like to inject something at a spesific offset prior to running an animation.

Example

Offsets must be given in incrementing sequence, spanning between 0 .. 1.

``js
import spreadOffset from "spread-offset"

let r = spreadOffset([
{value: "val"},
{value: "val"},
{value: "val"},
{value: "val"}
])

// results to

r = [
{value: "val", offset: 0},
{value: "val", offset: 0.333333333333},
{value: "val", offset: 0.666666666666},
{value: "val", offset: 1}
]
`

Spread offset respects existing offset declarations, and interpolates around them.

`js
let r = spreadOffset([
{value: "val"},
{value: "val"},
{value: "val", offset: 0.5},
{value: "val"}
])

// results to

r = [
{value: "val", offset: 0},
{value: "val", offset: 0.25},
{value: "val", offset: 0.5},
{value: "val", offset: 1}
]
``

Conribute

All feedback is appreciated. Create a pull request or write an issue.