Basic wgpu renderer

[hecrj]

This PR sets up the foundations for:

• iced_wgpu, a renderer for Iced on top of wgpu. Fixes Implement a low-level renderer #8.
• iced_winit, a renderer-agnostic runtime and conversion functions for winit.
• iced, a batteries-included cross-platform GUI library using iced_wgpu, iced_winit, and iced_web.

As a consequence, the tour example depends only on iced now. All its previous direct dependencies, including ggez, have been dropped and the whole example fits in a single file now! 🎉

Implementation details

The most important new concepts are:

• The iced_native::Renderer trait with an Output associated type. A Widget returns a value of this type in Widget::draw, instead of a MouseCursor.
• The iced_native::renderer::Windowed trait, which uses raw-window-handle to define a reusable interface for graphical renderers.
• The Application traits in both iced_winit and iced, which can be directly used to implement a GUI application easily. It should be a matter of implementing the trait and calling its run method! The iced_winit one allows users to choose a renderer with its Renderer associated type, while the iced one chooses iced_wgpu as the default renderer on native platforms (at least for now).

iced_wgpu

iced_wgpu implements the new Renderer and Windowed traits and defines its Output as (Primitive, MouseCursor). The Primitive type in iced_wgpu is a simple enum listing the different supported primitives. Currently, three primitives are supported:

1. Text with a size, color, alignment, and specific boundaries. wgpu_glyph is used to render primitives of this kind.
2. Quad with a background color (gradient and image backgrounds are not done yet!), border radius, and some boundaries. A pipeline with a simple fragment shader is used to render multiple instances of this primitive at once.
3. Group, a group of primitives.

UserInterface::draw can be used to obtain a tree describing the different primitives that comprise the GUI. These primitives can be then drawn in the next frame using Renderer::draw.

iced_winit

iced_winit implements a renderer-agnostic runtime on top of winit in its Application trait. This new trait can be implemented by simply providing Renderer and Message types, and implementing the update and view logic of the GUI. Once implemented, Application::run can be called, which will take control of the current thread, create a window, and run the application.

It is important to note that using the new Application trait is optional. winit users can still build their own event loop and use iced_native::UserInterface directly. For this purpose, a conversion module is exposed and can be used to convert winit events into iced_native events easily.

Changelog

Added

• iced_wgpu crate, a simple renderer for iced_native on top of wgpu.
• iced_winit crate, a renderer-agnostic runtime for iced_native on top of winit.
• Renderer trait in iced_native with an Output associated type.
• renderer::Windowed trait in iced_native, which defines a reusable interface for graphical renderers thanks to raw-window-handle.
• Background type in iced_core, it only supports a Color variant for now.
• Button::padding, a method to define the padding of a button.
• Button::background, a method to define the Background of a button.
• Button::border_radius, a method for displaying buttons with rounded borders.
• Color::WHITE constant.
• Color::into_linear, which converts a Color into its linear values.

Changed

• Button supports any element as content, instead of just a label.
• Widget::draw returns a Renderer::Output.
• Every widget Renderer extends the new iced_native::Renderer trait.
• Image does not have generic handle now, it always takes a String representing a path or URI.

Removed

• button::Class enum. Given that buttons are now customizable, it is no longer necessary.
• Button::class method.

Pending work before merge

• Implement the Image primitive. It's the only missing piece to make the tour work as before.
• Help finish Tracking issue: Rethinking RedrawRequested  rust-windowing/winit#1082 on macOS. The iced_winit runtime relies on the new RedrawRequested API to handle resizes properly. As of now, it only compiles on Windows and Linux with X11.
  • Implement revamped RedrawRequested on macOS rust-windowing/winit#1235
• Make the tour example work on web using wasm-bindgen.