Add trailing_fill() toggle to Slider

[hinto-janai]

Adds a boolean toggle to Slider that enables trailing color behind the circle. The color used is the same as a ProgressBar, it is sourced from ui.visuals().selection.bg_fill.

Usage:

ui.add(Slider::new(&mut my_slider_value, 0.0..=100.0));                      // No-color (default)
ui.add(Slider::new(&mut my_slider_value, 0.0..=100.0).trailing_color(true)); // Trailing color

Before:

After:

Vertical & Horizontal:

[emilk]

This is a very nice feature!

But I wonder if we can come up with a better name for it. trailing_fill?

I also think the option belong in egui::Visuals instead of (or addition to) having it for every slider. Presumably a user would want to turn this on for all sliders, or none of them?

[hinto-janai]

Suggestions added. The option is now in egui::Visuals with a per Slider override:

ui.add(Slider::new(&mut my_slider_value, 0.0..=100.0));                      // No fill (default)
ui.add(Slider::new(&mut my_slider_value, 0.0..=100.0).trailing_fill(true));  // Fill (override)

ui.visuals_mut().slider_trailing_fill = true;

ui.add(Slider::new(&mut my_slider_value, 0.0..=100.0));                      // Fill (global)
ui.add(Slider::new(&mut my_slider_value, 0.0..=100.0).trailing_fill(false)); // No fill (override)

The ui.painter() code was copied from the code that was already there, so I changed its style as well.

[emilk]

thanks!