KSES: Allow filter property to accept a URL in safecss_filter_attr().

CSS filters can accept `url()` as a reference to an SVG filter element:
{{{
filter: url( file.svg#filter-element-id );
}}}
This commit allows for that syntax to be used in inline CSS.

Original PR from Gutenberg repository:
* [https://github.com/WordPress/gutenberg/pull/48281 #48281 Duotone: Use the style engine to generate CSS for Duotone]

References:
* [https://developer.mozilla.org/en-US/docs/Web/CSS/filter MDN Web Docs: filter()]
* [https://developer.mozilla.org/en-US/docs/Web/CSS/url MDN Web Docs: url()]

Follow-up to [44136], [52049].

Props scruffian, jeryj, ironprogrammer, azaozz, hellofromTonya, SergeyBiryukov.
Fixes #57780.

git-svn-id: https://develop.svn.wordpress.org/trunk@55564 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/kses.php

@@ -2279,6 +2279,7 @@ function kses_init() {
  *              Extended `margin-*` and `padding-*` support for logical properties.
  * @since 6.2.0 Added support for `aspect-ratio`, `position`, `top`, `right`, `bottom`, `left`,
  *              and `z-index` CSS properties.
+ * @since 6.3.0 Extended support for `filter` to accept a URL.
  *
  * @param string $css        A string of CSS rules.
  * @param string $deprecated Not used.
@@ -2466,6 +2467,7 @@ function safecss_filter_attr( $css, $deprecated = '' ) {
 		'background-image',
 
 		'cursor',
+		'filter',
 
 		'list-style',
 		'list-style-image',

tests/phpunit/tests/kses.php

@@ -1316,6 +1316,11 @@ EOF;
 				'css'      => 'aspect-ratio: url( https://wordpress.org/wp-content/uploads/aspect-ratio.jpg );',
 				'expected' => '',
 			),
+			// URL support for `filter` introduced in 6.3.
+			array(
+				'css'      => 'filter: url( my-file.svg#svg-blur );',
+				'expected' => 'filter: url( my-file.svg#svg-blur )',
+			),
 		);
 	}