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/

graphics/src/lib.rs

@@ -27,6 +27,7 @@ mod transformation;
 mod viewport;
 
 pub mod backend;
+pub mod color;
 pub mod compositor;
 pub mod damage;
 pub mod gradient;