GIF decoding: do not seek() if the image is loaded over the network

This suppresses the warning
QIODevice::seek: Cannot call seek on a sequential device

Task-number: QTBUG-39217
Change-Id: Ie7b0845c760ae6fc857d02bf9ec5c5adb24fb631
Reviewed-by: aavit <eirik.aavitsland@digia.com>
Doc: qInstallMessageHandler cannot return 0.

The commit 124044613dde9a9596312102ca377bc74ed08165 (in Nov 2011) changed
that: the first call will return the builtin message handler, not 0.

Change-Id: I535ad69639f2341f9b664a6e2e7b12802ae785e0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Make the fetchAndAddRelaxed function a member template

This way, no compiler can instantiate it at class instantiation time. We
don't want them to do it for T that are function pointers (sizeof
functions is meaningless).

Change-Id: I6d5044bd5d9ffd0d347f1f38ab33c64213730788
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Fix QRingBuffer::readPointerAtPosition()

Fix condition to allow return a valid pointer when head != 0.

Change-Id: I5215f7dfc44924016c2d9b67ab2d9935b5164d7a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>