Lyon GPU-accelerated canvas

[dhardy]

The recently added canvas (#215) uses CPU rendering via tiny-skia to a cached texture.

Tiny-skia does not support GPU rendering at all; Lyon provides a vaguely similar API and does; see this example.

The objective should be an easy-to-use Lyon Canvas widget over a fixed pipeline (or fixed set of pipelines). This requires an API which doesn't exist in the examples, but could work roughly as follows:

// add to DrawSharedT:
// this method tessellates a path, returning some identifier
#[cfg(feature = "lyon")]
fn lyon_path(&mut self, path: lyon::path::Path, ..) -> PathId;

// new trait:
#[cfg(feature = "lyon")]
pub trait DrawableLyon: Drawable {
    fn lyon_instances(&mut self, instances: &[Primitive]);
}

It is an open question how flexible the API should be. Note that a custom WGPU pipeline (similar to the Mandlebrot example) can be used where this API insufficient.