CssOptimizer should leave url('#hash') alone

Problem/Motivation

\Drupal\Core\Asset\CssOptimizer anchors all paths in the CSS with its base url. Aside of ignoring external and absolute paths, it should also ignore paths starting with a hash (#).

Steps to reproduce

Add urls in a CSS file which contains a hash sign, for example:




.blur {
  filter: url(my-file.svg#svg-blur); /* the URL of an SVG file used as a filter */
}

.inline-blur {
  filter: url(#svg-blur); /* the ID of an SVG that is embedded in the HTML page */
}

Proposed resolution

Change the regex so #hash urls will be left alone. See the patch in #9.

Remaining tasks

Review the patch and commit it to core.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

None.