Re: [w3c/manifest] Add a 'masking' purpose, for allowing the UA to supercompose such icons (#657)

You're correct that all the pixels are supposed to be opaque in a maskable icon:

> If the icon contains transparent pixels, the user agent MUST composite the icon onto a solid color (eg. white) of the user agent's choice.
>
> NOTE: It is suggested that designers avoid using transparent pixels in maskable icons.

The idea is that you supply a "full-bleed" square icon with no transparent pixels.

When you say "it would be difficult for user-agents to add additional padding"... I guess you mean if the icon was a non-square icon with transparent pixels, you can just extend the transparency outwards. But with a "full-bleed" square icon, how do you do that? This is true, but we figured the user agent could just extend it out with black pixels, which will never be seen because Android masks out the background and doesn't parallax it (it only parallaxes the foreground). To be very sure, the user agent could extend the pixels at the edge of the image outwards, instead of just using black.

Again, see my [July 11 post](https://github.com/w3c/manifest/issues/555#issuecomment-404097653), where one of the images has a thick black border around it; that simulates the Android user agent's extending the image to match Android's required dimensions.

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