Add high density display support

[SimonBackx]

For now, when you specify a size of 320 pixels, it will only generate an image with a size of 320px. But most smartphones have higher density displays, so we also need to add 640px (x2) and 960px (x3) to the srcset attribute (not in the sizes attribute). Adding those sizes to the currently available 'sizes' options, is not a possibility since that will also include it in the sizes attribute (which is not allowed, since that would generate incorrect media conditions)

[RobbieTheWagner]

Any updates on this? We really need a way to support 2x and 3x in this package!

[jfbloom22]

I was well on my way integrating this plugin, noticed the quality issue on my 4k 24" monitor and am now removing it. Will be back when high density is supported.

[KaragiannidesAgapios]

Any updates on this?

[northamerican]

why was this closed? we still seem to be missing hidpi display support.