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/Cargo.toml

@@ -11,6 +11,7 @@ repository = "https://github.com/iced-rs/iced"
 geometry = ["iced_graphics/geometry", "lyon"]
 image = ["iced_graphics/image"]
 svg = ["resvg"]
+web-colors = ["iced_graphics/web-colors"]
 
 [dependencies]
 wgpu = "0.16"