Bootstrap: Enhance core's memory limit handling.

* Don't lower memory limit if the current limit is greater than `WP_MAX_MEMORY_LIMIT`.
* Set `WP_MEMORY_LIMIT` and `WP_MAX_MEMORY_LIMIT` to current limit if the `memory_limit` setting can't be changed at runtime.
* Use `wp_convert_hr_to_bytes()` when parsing the value of the `memory_limit` setting because it can be a shorthand or an integer value.
* Introduce `wp_raise_memory_limit( $context )` to raise the PHP memory limit for memory intensive processes. This DRYs up some logic and includes the existing `admin_memory_limit` and `image_memory_limit` filters. The function can also be used for custom contexts, the `{$context}_memory_limit` filter allows to customize the limit.
* Introduce `wp_is_ini_value_changeable( $setting )` to determine whether a PHP ini value is changeable at runtime.
* Remove a `function_exists( 'memory_get_usage' )` check. Since PHP 5.2.1 support for memory limit is always enabled.

Related commits: [38011-38013]

Props jrf, A5hleyRich, swissspidy, ocean90.
Fixes #32075.

git-svn-id: https://develop.svn.wordpress.org/trunk@38015 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/image-edit.php

@@ -585,8 +585,7 @@ function image_edit_apply_changes( $image, $changes ) {
 function stream_preview_image( $post_id ) {
 	$post = get_post( $post_id );
 
-	/** This filter is documented in wp-admin/admin.php */
-	@ini_set( 'memory_limit', apply_filters( 'admin_memory_limit', WP_MAX_MEMORY_LIMIT ) );
+	wp_raise_memory_limit( 'admin' );
 
 	$img = wp_get_image_editor( _load_image_to_edit_path( $post_id ) );