Comment 5 for bug 444424

Fix committed.

Looks like it was as mentioned in above comment, _process_element did transverse up the element tree when moving past the end of an element, but didn't process those element using a pixelate.

I've put a small test case into tests.py to test for this behaviour, and checked against the existing tests, looks good but should be tested against the newer tests to confirm it doesn't break existing behaviour.