This splits out the image-specific optimizations from Optimization Detective into a separate dependent plugin: Image Prioritizer.

Please review the commits one-by-one as they have been tailored to be as atomic as possible. Also, suppressing whitespace changes will help with reviewing methods for which caching was added.

The image optimization logic previously located in the od_optimize_template_output_buffer() function has been heavily refactored in the following ways:

A new OD_Tag_Visitor_Registry class has been introduced which allows plugins at the od_register_tag_visitors action to register callbacks that are invoked for each tag while iterating over the document. This callback is passed the OD_HTML_Tag_Walker instance, allowing the callback to inspect the current tag and attributes, as well as to obtain the XPath. With this information in hand, it is able to look up the element in the provided OD_URL_Metrics_Group_Collection instance to see if it is an LCP element (or in future, whether it was in the viewport, etc). It can also add preload links via the provided OD_Preload_Link_Collection instance. If the callback returns true then this indicates that the tag is relevant for the visitor. Such visitor-related tags will automatically get the data-od-xpath attributes added to them when the collection of URL metric groups is not complete; this ensures that the element is captured in the URL metrics.

Here's a basic example of how a visitor could be implemented with a closure:

<?php
add_action(
	'od_register_tag_visitors',
	function ( OD_Tag_Visitor_Registry $tag_visitor_registry ): void {
		$tag_visitor_registry->register(
			'img-dimensions',
			function ( OD_HTML_Tag_Walker $walker ): bool {
				if ( $walker->get_tag() !== 'IMG' ) {
					return false;
				}

				$src = trim( (string) $walker->get_attribute( 'src' ) );
				if ( ! str_starts_with( $src, 'http://' ) && ! str_starts_with( $src, 'https://' ) ) {
					return false;
				}

				$width  = (int) $walker->get_attribute( 'width' );
				$height = (int) $walker->get_attribute( 'height' );
				if ( 0 !== $width && 0 !== $height ) {
					return false;
				}

				// Example code to fetch dimensions (but without transient caching 🙈).
				$client = new \FasterImage\FasterImage();
				$images = $client->batch( array( $src ) );
				list( $width, $height ) = current( $images )['size'];

				$walker->set_attribute( 'width', (string) $width );
				$walker->set_attribute( 'height', (string) $height );

				return true;
			}
		);
	}
);

The image optimization logic is further refactored into two classes:

• IP_Img_Tag_Visitor handles optimizing LCP img tags
• IP_Background_Image_Styled_Tag_Visitor handles optimizing LCP elements with a background-image style.

Both of these classes inherit from an IP_Tag_Visitor abstract class that sets up the common interface. Note that these classes implement an __invoke() method, allowing them to be passed to the tag visitor registry to be called the same way as a closure.

Fixes #1088