Preview of upcoming egui dark-on-white text rendering improvements

[barries]

I saw this commit went in and got excited... here's how it looks in my most text-y app.

• (HEAD also made the width of a ui.label(" ") far narrower. Not sure if that's a bug or a fix, but I switched to ui.add_space(5.0 * char_width), which is the correct approach. I'm not sure why I had that cheazy hack / workaround in place, but it's gone now. I know I've had trouble with ui.add_space() not seeming to work in some layout combinations, but it seems to work better now in this one place, anyway.