resourceloader: Make various CSSMin performance optimizations and cleanups

resourceloader: Make various CSSMin performance optimizations and cleanups

This is called relatively often. Even small improvements might have an
impact.

I'm intentionally replacing method_exists with class_exists because the
old check looked like it was done for backwards compatibility (MediaWiki
before 1.27 did not contained the method), while in reality this code is
meant to run without MediaWiki. This is much better reflected with a
straight "if this class doesn't exist, there is no MediaWiki".

I'm intentionally using the …::class feature. Yes, this works, even if the
class is not there.

Change-Id: I7f250a7cb000105bb751f68f25c6cc1c44c8f221