Since 5.7, QIODevice::peek() implementation is based on transaction
mechanism. While technically it's correct, seeking backward on a
buffered random-access device clears the internal buffer that affects
the performance of reading.
To solve the problem, this patch implements peek mode directly inside
the reading procedure.
Task-number: QTBUG-56032
Change-Id: Ic5269f76e44c491a0309e13aba87fa7cf7b9259f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
6e8fcab7e0