Relax too strict life times on ChartBuilder

[jchacks]

The ChartBuilder's lifetimes meant that the root_area was not allowed to be dropped after the builder has been finalised (i.e. with a call to build_cartesian_2d). I'm not 100% sure but this feels like unintended behaviour.

Since the root_area is cloned and no references to the original (usually root DrawingArea) are maintained by the resulting ChartContext this lifetime can be relaxed.

This should allow for ChartContexts to outlive the root drawing areas.

let drawing_area = create_mocked_drawing_area(640, 480, |_| {});
let mut chart = ChartBuilder::on(&drawing_area)
    .build_cartesian_2d(0f32..10f32, (1e-6f32..1f32).log_scale())
    .unwrap();
drop(drawing_area); // This drop would not be allowed before
// do something with `chart`