Support clipping in DrawingBackend

[Zoxc]

Currently drawing outside the chart area is handled incorrectly because the drawing backend has no clipping support. To fix this we could clip drawing to the chart area and no longer truncate coords in CoordMapper impls.

[bluenote10]

I guess this issue refers to the following problem:

use plotters::prelude::*;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let root = BitMapBackend::new("basic_plot.png", (640, 480)).into_drawing_area();

    root.fill(&WHITE)?;
    let mut chart = ChartBuilder::on(&root)
        .caption("y=x^2", ("sans-serif", 50).into_font())
        .margin(5)
        .x_label_area_size(30)
        .y_label_area_size(30)
        .build_cartesian_2d(-2f32..2f32, -0.1f32..1f32)?;

    chart.configure_mesh().draw()?;

    chart
        .draw_series(LineSeries::new(
            (-100..=100).map(|x| x as f32 / 50.0).map(|x| (x, x * x)),
            &RED,
        ))?
        .label("y = x^2")
        .legend(|(x, y)| PathElement::new(vec![(x, y), (x + 20, y)], &RED));

    chart
        .configure_series_labels()
        .background_style(&WHITE.mix(0.8))
        .border_style(&BLACK)
        .draw()?;

    root.present()?;

    Ok(())
}

Produces: