BIP 0009: Remove transparent background from figure.

Dear champion, authors, community:

A copy of the revision message follows:

Remove transparent background from figure.

Before this change, the figure presented black text on transparent background, which might be unconvenient when using a browser able to pass a dark theme preference to some environments where this document is published, currently notably GitHub. A white background could help a better visualization compromise. White background on the figure is the single purpose of this revision.

This PNG was compiled using:

dot -Tpng states.gv -o states.png

TL;DR: I have some visualization problems on dark theme, and this fixes it for me.

TS;DR: Please ask for anything requiring explanation, screenshots showing the problem, visualization changes... anything you need.

Thank you.

Cheers,