Use WebP image in templates

[jjgrainger]

A couple of support tickets have been raised regarding using the WebP version of images in templates.

Currently, the plugin replaces images URLs to their WebP version in the WordPress content only.

Providing a way to use WebP outside of the content has been explored as part of a core patch (#155) but has been paused due to feedback on the wider WebP core patch.

As an initial step towards supporting WebP across templates, we could use hooks to modify the returned URL by core functions. As a starting point we can explore using the wp_get_attachment_image_src hook.

This could be optional by providing a custom filter in the plugin to allow developers to enable WebP support outside of the content. The filter could accept an array to provide more granular control over what image sizes should be returned as WebP if needed.

[bethanylang]

Tagging in @felixarntz and @adamsilverstein here for review. I agree based on what we've seen on the support forums that this is something that we should address at some point.

[felixarntz]

This is related to #523, however the two are a bit different.

• I think #523 is a safer first step because it only touches a function that returns an img tag, not just an image URL. We should probably expand that issue to also cover tweaking behavior of other core functions than wp_get_attachment_image() that return an img tag (e.g. get_the_post_thumbnail()).
• We have to keep in mind that not every image ever used by anything in WordPress should be a WebP as there are scenarios where a JPEG needs to be requested. Therefore I am wary of changing lower level functions like wp_get_attachment_image_src() by default.
• Of course those lower level functions should be changed so that the caller can control which MIME type to request, but that is unfortunately not something we can do until it gets into core. We could think about ways though to "opt in" to getting WebP from these functions, maybe even create a wrapper function like perflab_get_attachment_image_src_webp() that temporarily adds a filter then calls wp_get_attachment_image_src() which would then return a WebP image and its data - just some ideas.

@bethanylang Regarding the support requests around this, I believe they should notably become less once we have released #524, as then we basically go "all in" on WebP. This also means that issues like this one and #523 may be less urgent to address. We should still work on them probably, but I just want to point that out.