Fill plot area when using min/max down sampling

[CramBL]

Can be achieved by painting trapezoids between the plots like this:

fn create_filled_segments(line1: &[[f64; 2]], line2: &[[f64; 2]]) -> Vec<Polygon> {
    let mut segments = Vec::new();

    for i in 0..(line1.len() - 1) {
        // Create a trapezoid between line1[i], line1[i+1], line2[i+1], and line2[i]
        let mut points = Vec::new();
        points.push(line1[i]);
        points.push(line1[i + 1]);
        points.push(line2[i + 1]);
        points.push(line2[i]);

        // Create a polygon for this segment
        let polygon = Polygon::new(PlotPoints::new(points))
            .fill_color(Color32::from_rgba_premultiplied(100, 40, 50, 0))
            .stroke(Stroke::NONE);
        segments.push(polygon);
    }

    segments
}

[CramBL]

This is more complicated that it seems... Polygons has to be convex, and drawing it like the approach above ends up painting areas outside the min/max area between plots.

Going with a triangles approach also doesn't work, if the triangle has one very large angle and two small then visual artifacts appear that stretch the corner with small angles beyond the points where they actually end.

[CramBL]

The issue is already reported at egui_plot: emilk/egui_plot#36