grafx - Extensible Java Graphics Package Code

Making a comment change to ffmpeg-overlay.sh. On every change I commit the
same changes to multiple repositories, because this file is used by multiple
SVN projects, unfortunately. I find this script to be extremely useful in
simplifying complex tasks to be much more easily understood.

Fixing the issue in ffmpeg-overlay.sh with absolute paths and underscores.

Adding another TODO.txt related to ffmpeg-overlay.sh.

Adding another TODO.txt item related to examination of GoPro video FPS.

Adding missing line 'echo "${DURATION}"' in duration() function in
ffmpeg-overlay.sh.

Defining function 'duration' which performs the ffprobe command that used to be
FFPROBE_CMD.

Revising TODO.txt with some items that are done, and adding a new item related
to end of level incentives.

Making a mega comment update on scripts/vid/ffmpeg-overlay.sh, and skipping
completely blank lines in input file; treating those as comments.

Making major improvements to ffmpeg-overlay.sh. I still need to test a few
items, and I need to document more clearly.

The third in the series of changes, please see previous comment. Allowing
FFmpeg modifier to be passed to source video, sub video, overlay image, and
output video.