Use the same default style for both builders.

[xStrom]

The two builders (ranged/tree) use different sources of truth to achieve a default style. This is already a bad idea, as evidenced by them having a different default line height until very recently. It gets even worse, because the ranged builder uses ResolvedStyle::default() which means it has a dummy font and doesn't respect scale.

This PR makes both builders use a single source of truth - a TextStyle which then gets resolved.

This means that we can confidently change our default style in only one place - the TextStyle::default method. Those default values then also get properly resolved according to available fonts, scale, etc.

Note that I didn't change the public API on purpose. I do think that being able to immediately provide a custom root style to the ranged builder, like it is possible with the tree builder, is probably worth doing. However, I want to do any potential public API changes as a focused follow-up PR.

The PlainEditor::ime_cursor_area method needed a slight modification as well. It was depending on ResolvedStyle::default() for a font size based calculation. That did not respect scale, so I fixed it by just keeping track of the font size that was provided to the PlainEditor constructor.

I replaced the custom ResolvedStyle::default() implementation with a simple derive. The only remaining non-derivable part was setting the font size to 16. Given that it doesn't respect scale, that custom implementation just provides a false sense of utility. A zero font size is perfectly fine for a state clearing default, which is how we're still using it.

I also added a test to ensure that the default styles remain in sync between the different builders.

[DJMcNab]

Approving if and only if we still get rendering if you don't explicitly set a font size. Having a silent non-rendering due to the default font size being 0 would be an extremely poor UX (I could also forsee it leading to other fun bugs, such as with the overlapping line heights, although maybe we already addressed that).

[xStrom]

There is no API surface that actually uses the 0 font size. It's only being used internally for clearing memory. 👍