Remove LazyLoad CSS Background Images script from not processed pages

[piotrbak]

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Describe the bug
When there's no WP Rocket buffer involved in processing the page, we should not inject the script

To Reproduce
Steps to reproduce the behavior:

1. Enable LazyLoad CSS Background Images feature
2. Add define('DONOTCACHEPAGE', true) to wp config
3. Go to the website in incognito mode and check the source
4. The script is there

Expected behavior
When buffer is not involved, the script should not be added

Acceptance Criteria (for WP Media team use only)

1. The script should not be injected when define('DONOTCACHEPAGE', true) is set
2. The script should not be injected when the page is excluded from being processed (never cache URLs, or from the page editor point of view)
3. The script should be added when the cache is not generated but optimisations are there (add_filter( 'do_rocket_generate_caching_files', '__return_false' );)
4. The same goes for LazyLoad Images script, no regression here
5. The script should be added in regular circumstances

[CrochetFeve0251]

Reproduce the problem

The issue was easy to reproduce.

Identify the root cause

The root cause is that we are not checking DONOTCACHEPAGE constant inside should_lazyload from CanLazyloadTrait.

Scope a solution

To solve that we will have to check the DONOTCACHEPAGE at this level inside should_lazyload from CanLazyloadTrait.

To support excluded URI we can use the following logic inside the same method:

		global $wp;

		if ( is_user_logged_in() ) {
			return false;
		}

		$url = home_url( add_query_arg( [], $wp->request ) );
		$cache_reject_uri            = get_rocket_cache_reject_uri();
                if (! preg_match( "@$cache_reject_uri$@m", $url ) ) {
				return false;
			}

Then tests concerning lazyload will have to be modified and for that the code inside the lazyload not merge will help.

Estimate the effort

Effort XS