Introduce web-colors feature flag to enable sRGB linear blending

This is how browsers perform color management. They treat
gamma-corrected sRGB colors as if they were linear RGB.

Correctness aside, this mode is introduced for legacy reasons. Most
UI/UX tooling uses this color management as well, and many have created
an intuition about how color should behave from interacting with a
browser.

This feature flag should facilitate application development with `iced`
in those cases.

More details: https://webcolorisstillbroken.com/

wgpu/src/quad/solid.rs

@@ -1,3 +1,4 @@
+use crate::graphics::color;
 use crate::quad::{self, Quad};
 use crate::Buffer;
 
@@ -9,7 +10,7 @@ use std::ops::Range;
 #[repr(C)]
 pub struct Solid {
     /// The background color data of the quad.
-    pub color: [f32; 4],
+    pub color: color::Packed,
 
     /// The [`Quad`] data of the [`Solid`].
     pub quad: Quad,