comark/annotate/compare-screenshot-count · Give every ::compare two screenshots, before and after

A ::compare shows a before and an after, so it takes exactly two screenshots. With one, there's nothing to compare it to; with three or more, readers can't tell which pair to look at.

Incorrect

::compare
:::screenshot{src="/pond-after.png" alt="The pond with a fence"}
:::
::

lint-comark example.mdexample.md:1:1 comark/annotate/compare-screenshot-countcompare 1: needs exactly two screenshots, before and after (got 1).Add the missing one.✖ 1 problem

Correct

::compare
:::screenshot{src="/pond-before.png" alt="The pond with no fence"}
:::

:::screenshot{src="/pond-after.png" alt="The pond with a fence"}
:::
::

lint-comark example.md✔ No problems