add support for disabling anti-aliasing in BitmapBackend

[jonaspleyer]

Description

This PR arose from a personal need where I wanted to quickly draw some geometries but they had to be solid of identical color. In this scenario, anti-aliasing is actually undesirable. Since I saw no functionality to disable this, I added it.

The PR does not yet contain any tests. I would love to hear some feedback if this approach is reasonable.
The introduction of a new defaulted method fn use_anti_aliasing(&self) -> bool to the DrawingBackend trait would probably be classified as a minor change by Rust's semver-specs https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-parameter-default thus requiring a minor version bump. However, I am happy to hear any feedback on this.

Example Usage

let (x, y) = ..;
let mut buffer = vec![0u8; x*y*3];
BitMapBackend::with_buffer(&mut buffer, (x, y))
    .anti_aliasing(false)
    .into_drawing_area();