change video extract default output extension from `png` to `jpg`

[mightbesimon]

In most cases, jpg is the preferred output extension for extraction. Even the help message itself states:

png is lossless, but extraction is x10 slower for HDD, requires x10 more disk space than jpg.

I think if the odd cases that require lossless png, they should require the additional step rather than have most cases require the extra step. This will also make bulk extraction easier too. Let me know if you agree with this perspective.

[mightbesimon]

Whoops, I didn't mean to push my additional modifications (batch frame extract). Reverted back to intended.

[marshalleq]

png doesn't have to be lossless though does it?