While it is not explicitly documented I believe GScanner::value and next_value
are meant to be considered read only.  gtk_rc_parse_style() shouldn't modify the
value directly but make a copy first, like is done in gtk_rc_parse_statement()

Motivation for considering the value read only is that then one can implement a
pre-parsed mmappable cache and all strings could point in the cache directly and
avoid extra copies.