Use maximum ascent/descent/leading from fallback fonts in shaping

Use maximum ascent/descent/leading from fallback fonts in shaping

When shaping a QScriptItem with a multi font engine, currently we
only take the ascent/descent/leading from the primary (first) font
engine in that multi font engine, however, subsequent engines used
during shaping may have larger ascent/descent/leading, disregarding
them may cause clipping issues in some cases.

It's fixed by checking each font engine used in the shaping process
and take the maximum value instead of the first one. On ATSUI we
merely make it compile.

Task-number: QTBUG-16719
Reviewed-by: Eskil
(cherry picked from commit c501403cb5a0c9ec21b00e0c2f640ae85566e0cf)