Implement gradient color interpolation option (#548)

* Added the better color averaging code (tested & functional)

* rustfmt

* Make Color f32 0..1, clarify premul/unpremul

* Fix imports and test name

* Premultiply gradient colors matching CSS

* Fix indentation

* fixup

* Add gradient image

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Co-authored-by: K's Thinkpad <K.T.Kraft@protonmail.com>

resources/default-config.kdl

@@ -153,6 +153,7 @@ layout {
         // The angle is the same as in linear-gradient, and is optional,
         // defaulting to 180 (top-to-bottom gradient).
         // You can use any CSS linear-gradient tool on the web to set these up.
+        // Changing the color space is also supported, check the wiki for more info.
         //
         // active-gradient from="#80c8ff" to="#bbddff" angle=45