feat: rgb() and rgba() support floating point numbers

[yisibl]

This PR will bring the rendering of rgb() and rgba() in resvg in line with modern browsers and specifications.

Previously, rgb(0, 127.8, 0) would be rendered as the black color of a fallback

The CSS Color L4 specification says:

contrary to CSS Color 3, the parameters of the rgb() function are of type <number>, not <integer>. Thus, any higher precision than eight bits is indicated with a fractional part.
https://drafts.csswg.org/css-color-4/#ex-hwb-serial

Closes: linebender/resvg#645

[RazrFalcon]

I guess it's fine as a temporary solution, but in the future we would probably have to store colors as f32.

[yisibl]

I guess it's fine as a temporary solution, but in the future we would probably have to store colors as f32.

Yes, I see a similar change in Chrome: https://chromium-review.googlesource.com/c/chromium/src/+/4598329

[yisibl]

Have you considered using the latest release of cssparser-color crate?
servo/rust-cssparser#363

[RazrFalcon]

It seems like cssparser-color still depends on cssparser and this is a very heavy dependency. No plans on using it.