Use DOMPDF_FONT_CACHE for new/remote fonts

Currently it does not seem like the `DOMPDF_FONT_CACHE` constant is used in a sensible manner. With this PR it's actually possible to use the fonts shipped with DOMPDF but to save new fonts and cache files into a separate writeable location. So `DOMPDF_FONT_DIR` only contains the provided fonts and does not have to be writeable.

For me it seems like this is what `DOMPDF_FONT_CACHE` would be supposed to be doing?!