The pen is designed to determine the bitmap size when omitted, which
helps users to see an image somehow even when they have no idea how the
arguments should be passed. And I realised that I didn't give enough
thought to rotate/skew transforms in former PRs. This commit fixes the
calculation of the bbox after transformation. Also tries to clear up how
the autosizing options work in the docstring.
Some minor fixes will follow.
a27789c477