Disable GSUB feature when text contains surrogates

Disable GSUB feature when text contains surrogates

This is a work-around for a feature missing in Harfbuzz that
can make a text run that contains a surrogate and a ligature
crash. This will potentially cause the ligatures to break up
if you combine them with a surrogate, causing visual changes
to the text, but the scripts that require GSUB should not be
affected by this, since they will not use surrogates. Still,
it's not a permanent fix, but will serve as a bandaid for the
crash until the underlying problem has been fixed.

Task-number: QTBUG-22275
Change-Id: I90c37fba76bc7d1f369f3afddd1bd0dc306f5750
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>