Web worker to retrieve image data from Orthanc

This web worker receives the instanceNumbers (1, 5, 9, etc) and corresponding instanceUUID from viewer.php. It then retrieves the image data by calling on pngToGray.php using the instanceUUIDs and postMessages the image data to viewer.php.

This is one of 4 web workers running (the others are named Even (for 2, 6, 10, etc), OddTwo (3, 7, 11, etc) and evenTwo (4, 8, 12, etc)