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

[valpackett]

wgpu would currently ignore the alpha: gfx-rs/wgpu#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 — on wayland at least.

---

for testing:

--- i/examples/counter/src/main.rs
+++ w/examples/counter/src/main.rs
@@ -1,7 +1,12 @@
-use iced::{button, Align, Button, Column, Element, Sandbox, Settings, Text};
+use iced::{
+    button, Align, Button, Color, Column, Element, Sandbox, Settings, Text,
+};

 pub fn main() {
-    Counter::run(Settings::default())
+    Counter::run(Settings {
+        background_color: Color::new(0.1, 0.2, 0.3, 0.3),
+        ..Settings::default()
+    })
 }

 #[derive(Default)]

Screenshot showing both patched wgpu and glow with the same too-bright color (this is not how 0.1,0.2,0.3 is supposed to look like) as a result of not premultiplying alpha:

Of course for Color::TRANSPARENT everything is fine.

[hecrj]

At the very least, this adds support for changing the opaque background color.

We can iterate further to properly support transparency. Thanks!

[hecrj]

Hmm... After thinking a bit more about it, I believe the background color should be able to change at runtime.

I imagine we could add it as an additional Application method. Thus, I am reverting this change for now.