Re: [w3c/manifest] icon density should be supported (#118)

@marcoscaceres What do you mean by "poisoned"?

So is the issue basically that people are confused by the difference between *size* and *density*? I agree, it is very confusing. Basically you could have a 32x32 icon base size. Then you could also have:

* A 64x64 icon, which is twice as large.
* A 32x32 icon at "2x" density, which is the same *size* but twice as *dense* as the base icon.

Both of these assets would be 64x64 pixel images, and in many (most?) cases they would in fact be the same image. But they have a subtle semantic difference that is the former is intended to be displayed larger on screen, so the former may contain more detail while the latter contains less.

Generalizing this, you realise that an "icon" is actually a 2D array of images, with size along one axis and density along the other. Is this the model that was originally provided by "density" in the manifest spec? I think providing a facility to supply both different sizes *and* different densities of the same icon is potentially too niche; almost all icons are going to look the same in "large size" as they do in "high DPI", so only a single axis (that we already provide in the manifest) is sufficient.

@owencm 

> Specifically, they worry about some products that change their icon depending on the size it's to be displayed, so a 512x512 icon on a dense phone should be different to that on a low resolution computer.

Is there any case where it's inappropriate to just use a larger icon size on the dense phone? A 512x512 icon seems a bit extreme on a low-res PC (taking up half the screen). Perhaps a more realistic example is what I've given above: a site specifies a 32x32 icon and a 64x64 icon in its manifest. It needs to be displayed in a (logically) 32x32 square. On the standard-res PC, we just use the 32x32 asset. Is there any reason why we can't just use the 64x64 asset on the high-DPI phone, in the logically-32x32 square? Do we need to let the site specify a separate 32x32 high-DPI asset?

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