There was an explicit int cast in fromBase64Encoding() which was never
ported to qsizetype and therefore truncated the result.
Fix by removing the int cast.
Add a test, optimize it for as low memory usage as possible, given we
need to work in input and output data each in excess of 2GiB.
Fixes: QTBUG-104985
Pick-to: 6.4 6.3 6.2
Change-Id: I9c0924957e62e5cb3003132cd811b8b0315d8ac1
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
d76bf64531