What?

Currently, the Duotone metadata initialization runs on wp_loaded. This PR changes it to instead run the first time the metadata is needed.

Why?

Computing the Duotone metadata requires running WP_Theme_JSON_Resolver_Gutenberg::get_merged_data(), which performs a number of expensive operations across several large JSON tree structures that get loaded in from the filesystem. While WP_Theme_JSON_Resolver_Gutenberg::get_merged_data() relies on caching to reduce the amount of work in subsequent calls, it still takes a while to run the first time it's called for a pageview. Consequently, we want it to run as late as possible, so that we can exit before it does, if it's not needed.

This PR defers the Duotone metadata computations so that they happen on-demand. While this isn't enough to avoid the expensive work in most cases, it does mean that in some special cases (such as in some PHP early exit scenarios) they can be completely avoided. It also lays the groundwork for potential future improvements that avoid them in further scenarios.

How?

This PR adds two getters with the responsibility of initializing the internal class variables, and moves all class code to using the getters instead of accessing the variables directly.

Note that despite these changes, it's still possible to call the setter methods to explicitly initialize the variables, which means that any existing external code that happens to call WP_Duotone_Gutenberg::set_global_styles_presets or WP_Duotone_Gutenberg::set_global_style_block_names will continue to work correctly.

Note to reviewers: Since we can't rely on PHP 8.4's property hooks yet (which would be my preferred implementation), I decided to add a simple getter with the same name as the class variable, while prepending the class variable with __ to discourage direct use. I'm happy to consider any other approach, especially if I overlooked a more idiomatic technique.

Testing Instructions

Smoke-test the Duotone feature and ensure that it continues to work correctly, by adding a Duotone filter to an image block and ensuring the page loads correctly, with the filter correctly applied.