Add custom window background/clear color (incl. transparency) support, fixes #272

wgpu would currently ignore the alpha: https://github.com/gfx-rs/wgpu/issues/687

glow (and naively patched wgpu) requires premultiplied alpha, so if you don't
multiply the RGB by the A right now, the semi-transparent color would be wrong
(too bright).

winit with_transparent doesn't seem necessary.

wgpu/src/settings.rs

@@ -1,10 +1,11 @@
 //! Configure a renderer.
 pub use crate::Antialiasing;
+pub use iced_native::Color;
 
 /// The settings of a [`Renderer`].
 ///
 /// [`Renderer`]: ../struct.Renderer.html
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Settings {
     /// The output format of the [`Renderer`].
     ///
@@ -18,6 +19,9 @@ pub struct Settings {
 
     /// The antialiasing strategy that will be used for triangle primitives.
     pub antialiasing: Option<Antialiasing>,
+
+    /// The color that will be used to clear the window surface.
+    pub background_color: Color,
 }
 
 impl Default for Settings {
@@ -26,6 +30,7 @@ impl Default for Settings {
             format: wgpu::TextureFormat::Bgra8UnormSrgb,
             default_font: None,
             antialiasing: None,
+            background_color: Color::WHITE,
         }
     }
 }

wgpu/src/window/compositor.rs

@@ -119,10 +119,10 @@ impl iced_graphics::window::Compositor for Compositor {
                 load_op: wgpu::LoadOp::Clear,
                 store_op: wgpu::StoreOp::Store,
                 clear_color: wgpu::Color {
-                    r: 1.0,
-                    g: 1.0,
-                    b: 1.0,
-                    a: 1.0,
+                    r: self.settings.background_color.r.into(),
+                    g: self.settings.background_color.g.into(),
+                    b: self.settings.background_color.b.into(),
+                    a: self.settings.background_color.a.into(),
                 },
             }],
             depth_stencil_attachment: None,