STCC-219 fixed svg-to-png conversion error with xml register_namespace

[sandra0521]

This error occured when multiple svg conversions where running at the same time and were modifying the global dictionary matching namespace uri and their prefixes. Locking the call to register_namespace seems to have fixed the problem.

Testing is difficult as this error appeared randomly and cannot be reproduced on command. It appeared most often when converting projects with a large number of images.
How I did test: converting about 200 projects where I knew those errors occured multiple times in the past (from testing and using run_statistics) and this error did not appear again.

[oiduh]

I tested your solution with some scratch projects and everything worked fine. Then I did some research to find out why this lock is necessary: Apparently the function ET.register_namespace(prefix, uri) writes to a global dictionary, accessed by all the threads, hence the race condition.
I'm not sure if I'm wrong here, but would it be possible to create a global lock in the svgtopng.py file and use it instead of the one provided as parameter? If yes, all the changes in media_converter.py, test_converter.py and test_svgtopng.py wouldn't be necessary.