misc. bugfixes to generate peace love & understanding of the 2.4 branch

• bugfix for CSS minifier throwing a HTTP 500 error when a } is missing.
• bugfix for slowness when iconv was used for multibyte string replacements (ditched iconv).
• bugfix for occasional issues with broken/ missing strings due to mbstring (also for multibyte string replacement) weirdness.
• bugfix for wrong sizes in image optimization.
• bugfix for Google Font subset getting lost in optimization.