grafx - Extensible Java Graphics Package Code

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.

Second in a series of changes, please see previous log comment. This commit
may have some slight problems with the 'cut' command esp. when using
'--complement', which will be corrected in the next commit.