Cap height is an important metric of font, in particular it is
required to make decent implementation of "initial-letter"
CSS property in QtWebKit.
Note that some fonts lack cap height metadata, so we need to
fall back to measuring H letter height.
Change-Id: Icf69d92159d070889085e20d31f2e397d796d940
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
