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()
             };
 

src/settings.rs

@@ -1,8 +1,8 @@
 //! Configure your application.
-use crate::window;
+use crate::{window, Color};
 
 /// The settings of an application.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Settings<Flags> {
     /// The window settings.
     ///
@@ -32,6 +32,29 @@ pub struct Settings<Flags> {
     ///
     /// [`Canvas`]: ../widget/canvas/struct.Canvas.html
     pub antialiasing: bool,
+
+    /// The background color of the window.
+    ///
+    /// On supported backends, this makes it possible to have
+    /// (semi-)transparent windows.
+    ///
+    /// By default, it is white.
+    pub background_color: Color,
+}
+
+impl<Flags> Default for Settings<Flags>
+where
+    Flags: Default,
+{
+    fn default() -> Self {
+        Self {
+            flags: Default::default(),
+            antialiasing: Default::default(),
+            default_font: Default::default(),
+            window: Default::default(),
+            background_color: Color::WHITE,
+        }
+    }
 }
 
 impl<Flags> Settings<Flags> {
@@ -46,6 +69,7 @@ impl<Flags> Settings<Flags> {
             antialiasing: Default::default(),
             default_font: Default::default(),
             window: Default::default(),
+            background_color: Color::WHITE,
         }
     }
 }