Optimize QVector::mid

Change-Id: Iff7d9ec85a095c6712e6045e7708bb88eac629e1
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Ensure we don't re-use webfonts where the "local" font has been requested

For webfonts, we can't rely on the fontDef (and hence the cache Key) alone,
since the resulting fontcache key might be strictly identical.
Instead we have to check if the cached engine doesn't belong to a webfont
when we're looking for a "local" font.

Change-Id: I2de11c8fdbef53362b66674d3429a042e1835757
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>