Reduce the surface of the Renderer APIs

[hecrj]

This PR reduces the surface of the Renderer APIs in iced to simplify the process of implementing new renderers.

iced_graphics already offers a small set of Backend traits to implement renderers, but it is considerably opinionated as it forces their implementors to deal with a Primitive tree.

The changes here aim to move most of the logic from iced_graphics to iced_native by:

• unifying the Renderer traits,
• hiding the Primitive trees,
• and hardcoding the widget style sheets.

Unified Renderer traits

Instead of defining a Renderer trait for each widget, iced_native now defines only 4 main Renderer traits. Specifically:

• Renderer exposes methods to draw simple geometric shapes (just rectangles, for now!), as well as methods for layering and applying affine transformations.
• text::Renderer exposes methods to draw, measure, and hit-test text primitives.
• image::Renderer exposes methods to measure and draw raster graphics.
• svg::Renderer exposes methods to measure and draw vector graphics.

As a result, we can implement the drawing logic of the built-in widgets directly in the implementation of Widget::draw in iced_native, instead of depending on iced_graphics.

Hidden Primitive trees

The Primitive tree was initially introduced to allow for easy composition of rendering primitives. However, some renderer implementation may choose to use a different data structure to represent composable drawing operations and, as such, using Primitive should not be enforced.

Therefore, instead of outputting a Primitive as a result, Widget::draw expects you to record the draw commands directly into the renderer argument, using the Painter's algorithm.

Internally, the renderer can choose to represent these drawing operations as it pleases. iced_graphics may even be able to drop the Primitive tree altogether and generate the layers to be uploaded to the GPU directly during the recording of the draw operations!

Decoupled mouse::Interaction

Until now, Widget::draw expected us to produce a mouse::Interaction together with the widget Primitive. This was not only cumbersome but also incorrectly architected, since the mouse interaction has nothing to do with the drawing operations (see #377 (review)).

Therefore, the Widget and Overlay traits now feature a dedicated mouse_interaction method.

Hardcoded StyleSheet

Given that each widget now does not provide its own Renderer trait, the style of a widget is no longer defined through an assocciated type. As a consequence, we can directly depend on iced_style and hardcode the StyleSheet trait for each widget.

Furthermore, we can actually leverage lifetimes! Effectively, this means that now it is possible to borrow any Application state in any StyleSheet implementor!

Together, these changes remove a bunch of unnecessary indirection in iced_native, simplify the overall codebase, and allow for more freedom to implement custom renderers while reducing their API surface at the same time! 🎉