grafx - Extensible Java Graphics Package Code

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.

Making a series of three or four changes to scripts/vid/ffmpeg-overlay.sh which
will allow custom FFmpeg flags to be set on source videos (including base
video) and on output video. For example, scaling will be possible on all
source videos and images, and on the final output video. Much more will be
possible as well. This change simply introduces the '[N:v]null[videoN]' chain
terminology to overlay images (where 'N' is a number starting from zero) to
overlay images. This was already present in overlay video and on the base
video. The 'null' inside of this term will then be possibly modified with
scale properties etc.

Adding newer ffmpeg-overlay.sh script which has finer control of audio. Making
use of it in .compile-video-base.sh. Changing default level in
launch-computer-ai.sh. Adding 'set -e' flag in
combine-video-audio-shortest.sh.

Trying to fix lack of audio track in generated video.