Enable Optimize CSS files on performance page & Drupal dies

The way css is preprocessed at the moment, all @imports in a css file are loaded, optimised.

Note: This issue is actually by JoshuaRogers. It used to be:
#543892: Win XAMPP 1.7.2 Regex PHP Bug - Optimize css & long css comments fails

But that issue got changed to something else. This is separate.

Why is this a problem?
If a.css imports b.css which imports c.css, then c.css will be loaded and optimised. It will then replace the import statement in b.css. The resultant css (including the section that is already optimised) will be optimised, after which it will replace the import statement in a.css. At this point a.css (with the already optimised b.css and c.css) will be optimised before finally saving it. This means that many files may be processed multiple times with no benefit. In addition to this, it causes a large section of text to be sent to the regex (since all @imports are resolved.) This can cause a segfault in libpcre on some installations.

The patch?
Simply reorders the operations. The new order is optimise, fix the @imports. This means that each file is optimised only once, thus it keeps the size of the text sent to the regex from growing too large.