This just a collection of minor fixes to various things I noticed while looking over the code.
Adds a `Device::poll` call to the winit example.
Removes some lingering unnecessary `<f32>`s in the blit shaders.
Removes a duplicated bbox entry in the shared shader inclusion code and collapses nested `concat!`s.
Finally, it reintroduces the computation of separated alpha in the output of fine which I removed in a previous PR. This allows the `render_to_texture` mode to be useful for generating images that can be properly blended over other content. The blit shader has been changed to accommodate this.
This creates a new Renderer type that offers fairly simple render_to_texture and render_to_surface methods, the latter of which handles the final blit internally. Also adds a util module with some helpers for device and surface creation.
There is a new winit example in piet-wgsl/examples/winit that shows how to make use of it all. This should be fairly trivial to adapt to glazier/xilem.
