Implements case insensitive font family names.

One part (of 8!) of css font family disambiguation is that font families should
be matched case-insensitively.

This patch implements that. Once it lands, a bug needs to be filed to do lowercasing
properly (as a string, instead of char-by-char -- it's a unicode thing).

r? @glennw 
Oh crap I accidentally committed on top of my existing work! Sorry will fix.