Add inspector support for moof boxes
Create test cases for a moof box and sample mfhd, traf, and tfhd sub-boxes. Fix the source buffer codec so that the debug code on the mp4 muxer page works. The issue was that specifying an audio codec but then failing to include an audio track in the init segment generated a media decode error.
Adjust test page to allow segment-by-segment testing
Make the debug video and media source available in a larger scope. Every time a new segment is selected, forward they bytes into the active source buffer.