Image Block: Consider allowing folks to provide backup image formats

[kjellr]

This may generally better suited to a plugin (or CDN), but raising here for discussion anyway. Should we consider an advanced control that lets folks supply an alternate fallback images?

Here's a use case:

• A theme bundles WebP images, but would like to ensure support for older browsers.
• That theme bundles both WebP and JPEG images in its patterns.
• The image block serves whichever image is relevant to the end user.

If the image block outputted a picture element, it sounds like it could supply these fallbacks gracefully.

[desrosj]

Related: Add support for the picture element (Trac).