Implement offthread image rendering.

[paales]

Since this library is already pretty awesome and I thought it could use more awesome features: Render Images Off Thread.

Decoding images for use with a canvas is pretty common, whether it’s to allow users to customize an avatar, crop an image, or just zoom in on a picture. The problem with decoding images is that it can be CPU intensive, and that can sometimes mean jank or checkerboarding.

https://developers.google.com/web/updates/2016/03/createimagebitmap-in-chrome-50
https://github.com/GoogleChrome/offthread-image

Decoding an image Offthread will reduce scroll jank when loading the page because the main thread will have to do less.

Example how to implement in a custom element:
googlearchive/offthread-image#1
robdodson/offthread-image@9e517a0

I don't have any benchmarks so I can't say if this will actually improve performance, but it might be worth considering.

[mlisook]

Well, that is really cool.

I do want to maintain plastic-image as a superset of iron-image so there is a bit of complexity dovetailing with iron-image's 2 existing ways of displaying the image (img tag and background-image of the containing div) and adding a new way by inserting a canvas into the template. Also, iron-image has a fair amount of state tracking that plastic-image doesn't really have to deal with.

I certainly think it's worth an experimental build.

[mlisook]

@paales In case you missed it ... from the chomium project: Intent to Implement: image async attribute