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/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,