[Feature Request] Make IntoDrawingArea more flexible

[ecton]

What is the feature ?

IntoDrawingArea::into_drawing_area() has a return type that utilizes Self. This restricts into_drawing_area() from being used to only types that implement DrawingBackend. I propose using an associated type instead:

pub trait IntoDrawingArea: Sized {
  type Backend: DrawingBackend;

   fn into_drawing_area(self) -> DrawingArea<Self::Backend, Shift>;
}

This definition would enable implementations on any type that contains or can create a DrawingBackend type.

(Optional) Why this feature is useful and how people would use the feature ?

When adding support for plotters in both Kludgine and Cushy, I realized I can't implement IntoDrawingArea for my types, because the types that implement DrawingBackend aren't the types exposed to the end-user for rendering. The reasons vary by crate:

• In Kludgine, [Feature Request] Change estimate_text_size to take &mut self #535 required creating a wrapper that utilizes a RefCell. This wrapper type, not the renderer type, is what implements DrawingBackend.
• In Cushy, I purposely hide direct access to the renderer type to provide a limited API surface. Even if 535 were changed, Cushy would still have to provide a passthrough DrawingBackend implementation instead of being able to implement IntoDrawingArea to provide compatibility.

The current workaround in both of my situations is to have my own crate-specific as_plot_area() functions that return a DrawingArea. I would prefer to replace these APIs with implementations of IntoDrawingArea.