Now it's possible to quickly determine what calendar days have data and then
query recordings for just the day(s) of interest with their returned
{start,end}_time_usec.
I tested these in VLC and QuickTime. Both players appear to ignore the
as the track dimensions, track transformation matrix, box dimensions, and box
justification. I just left them at default values then.
Automated testing is minimal. There's a new test that the resulting .mp4
parses, but I didn't actually ensure correctness of the subtitles in any way.
