Ideally it would be nice if you could access everything, but it's not
part of the design of MS Paint, and I don't want to design for it here.
In MS Paint, there's a minimum window size.
* Implemented the Fill With Color tool using the algorithm described as
"combined-scan-and-fill span filler" on Wikipedia.
* I added handling for the affected region being None, which turned
out more complicated than I would like...
Some cases may be able to be simplified or removed.
* Also, I moved event.stop() to the top so I don't need to call it in
multiple places when there are multiple return points.
You could imagine having a brush with momentum, that swings around the
mouse without always reaching it. But it's just generally clearer to
not have an inadequate initial region that's then extended.
I mean, let's be honest, most of these symbols are garbage candidates.
But these ones I accidentally left from trying to get ChatGPT to find
emoji for me.
More helpful were https://emojidb.org/ and http://shapecatcher.com/
I wanted to avoid duplicating tool-related state between PaintApp and
Canvas, and prepare for adding different tools with more state and which
will want to live in a separate file.
This makes it slower, when running with `textual run --dev paint.py`;
when running with `python3 paint.py`, it's fine.
When running in dev mode with `textual console` devtool connected, it's
extremely much slower. But if it was faster, you'd have more messages
to scroll through, ha. So it's a tradeoff.*
*Ideally you want it to be fast and for the logs to be compacted.
In the future, I could bypass the message system for performance, but
for now I think it's better to stay idiomatic.
