Add a read position variable to eliminate excessive memcpy'ing when
reading a partial buffer.
Specifically, fix performance issue of reading large files from
QNetworkDiskCache in QtWebKit2.
Task-number: QTBUG-27522
Change-Id: I21edc909bf9223971b2c3db5f1fa6b89c5b61c5f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Antti Harju <antti.harju@ixonos.com>
