Improve image handling approach in Modern Image Formats

[adamsilverstein]

Feature Description

Switch from using the_content filter to using wp_calculate_image_srcset when replacing images.

As @joemcgill points out in #1236 (review) we should probably switch the image processing approach. Currently the webp_uploads_update_image_references function is attached to the_content filter and processes all images on the page using a regex. Instead, we can leverage the filters core runs on images after it does a similar regex against the content. Namely, the wp_calculate_image_srcset or wp_content_img_tag filters are potential places to add our logic.

[westonruter]

And note that webp_uploads_wrap_image_in_picture() is necessarily using the wp_content_img_tag filter.

[joemcgill]

Looking at this again today, I'd prefer using the wp_filter_content_tags filter as it would be the most straightforward change.

[b1ink0]

@joemcgill
I believe the wp_filter_content_tags filter does not actually exist in the core, but there is a function named wp_filter_content_tags() which processes images and iframes. As described in the doc comment for wp_filter_content_tags() function, it is intended to convert old HTML markup to new HTML5 markup ref.

Instead, I believe the wp_content_img_tag filter would be a better option. This filter is defined inside the wp_filter_content_tags() function, which already performs regex to find the img tags. Using this filter would eliminate the need for additional regex that we are doing in the webp_uploads_update_image_references function.