Removed ItemInfo::null

This should not be necessary. In QML the isNull property was nowhere
used, but anyway a comparison with null should work just fine.

On the C++ side, isNull was also not used, but in fact the returned
item is being checked against 0 for some debug warnings (which would
not have worked until now).

Finally, nobody owned this ItemInfo instance, and passing it to QML
would have risked it being deleted by the garbage collector.
Disabled clearing before rendering

We always expect the screen to be entirely covered, so this saves a few
GPU cycles.
Fixed bug in sprite loading

When an image set was encountered in a sprite definition, the case where
the image was already loaded was not handled correctly. Instead it would
wait indefinitely for a status change that already happened.