TGA-Reader: Bail out of check if the device is not seekable.

Suppress warning
'QIODevice::seek: Cannot call seek on a sequential device'
which is emitted further down when attempting to restore
the position of the file.

Task-number: QTBUG-31081
Change-Id: Ib6a4b3eecaa61905a29020e5dd428f1516bfba7f
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
test: Remove insignificant_test from tst_qtiff

This test is now passing on all CI platforms.

Task-number: QTBUG-28320
Change-Id: I6e0977980cfe788a087646eb751b113f83e8dd12
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>
Add changes-5.1.0 file

Change-Id: I07a808d9cf153e1cc0cfcfd2d5243b4dee8b3355
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>