Fix image allocation with Qt 6

To make the plugins fail to allocate if the image size is greater than QImageReader::allocationLimit() it is necessary to allocate the image with QImageIOHandler::allocateImage().

Note that not all plugins have been changed and some others are not tested in the CI (maybe due to missing libraries).

PS: the following message is printed by QImageIOHandler::allocateImage() if the size is exceeded: "qt.gui.imageio: QImageIOHandler: Rejecting image as it exceeds the current allocation limit of XXX megabytes"