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.
Surprisingly I found Preview.app can still display PGM images. While
somewhat legacy, it's a super straightforward format to (de)serialize.
The images are scaled to 50x50 px and only consume 5KB in total. Makes
more sense to human being than the previous base64-encoded zlib
compressed data, plus we don't have to add pillow as a dependency.
