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.

src/application.rs

@@ -195,6 +195,7 @@ pub trait Application: Sized {
                 } else {
                     None
                 },
+                background_color: settings.background_color,
                 ..crate::renderer::Settings::default()
             };