Enable GL context sharing for QtQuick windows by default

This is required for running QWebEngine inside qmlscene. It can be disabled
with --disable-context-sharing and it remains off in QtQuick. Instead apps have
to use QWebEngine API to enable this, where this patch here is merely
convenience.

Change-Id: I123893a7c8c644c49c66fe2b42d2b7b2e03622d9
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>