Support srcset in avatars.

[georgestephanis]

With the recent commits of [31107] and [31152] on #21195 we can more easily roll in native srcset support to avatars throughout WordPress! (yay responsive images)

Patch attached.

[pento]

I'm definitely in favour of adding srcset support, though I have a couple of reservations about this patch.

• Browser support is currently at ​~50%. I think we'd need to provide a shim for compatibility with all browsers. (I haven't investigated the current state of such shims.)
• The code for generating the sizes seems more appropriate as part of the ​responsive-images plugin. I don't really want to preempt the work done in that plugin, when we're relatively close to a global solution.

I'd like to see this land in the feature plugin first. If there are any extra filters required, we can probably add them in (either temporary or permanent, as needed).

[iseulde]

Can we keep this discussion in #22329?

[miqrogroove]

The biggest problem I see here is that the size and url args were both made filterable already. This patch assumes that the size arg corresponds to the 1x size for the url arg, which is no longer a valid assumption.

#31469 might be a step in the right direction, and even that might need to be expanded to keep all the different variables separated.

[iseulde]

Do we still need to do anything here after [31721]?

[miqrogroove]

Duplicate of #22329.

I suggest starting a new ticket if there are any stragglers.