vidcap: if n > 11, skips every 12th, 22nd, etc. image

[rootwork]

This line:

https://github.com/rootwork/bash-scripts/blob/main/videos/vidcap.sh#L307

is lazy. It works when there are fewer than 12 screencaps by simply skipping the 2nd (which is captured in the line prior and used as the top half image). But when there are 12 or more, it's going to skip every 12th, 22nd, 32nd, etc. image.

This should properly be a counter with a check when that counter equals (exactly) 2.