mp4mux: add gpmf track (Fixes #554)

This adds the cabability to insert telemetry data using the GoPro developed General Purpose Metadata Format (GPMF) into videos through mp4mux. A demo can be found here.

The telemetry data can be parsed from output videos using gpmf-parser. You can also use a number of video editing software, including Dashware and RaceRender, to add gauges and maps to the video using the embedded gpmf data. It also works with Quik, but the handler name of the video track will need to be "\tGoPro AVC" instead of the default "VideoHandler" in order for the videos to appear in the program. Here is a sample output from Quik.

I realize that this is a lot of code to support what seems like an edge case, but it seems like embedded metadata tracks is the future for video telemetry. There could be a separate plugin for video writing with the gpmf track, but I don't see how you are supposed to inherit/override the relevant sections of qtmux/mp4mux without copying the entire isomp4 folder. Another possible solution is to have support for qtmux/mp4mux to write a custom track where the user decides on the handler name, the caps and so on.