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

---------

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

niri-visual-tests/src/cases/layout.rs

@@ -50,8 +50,8 @@ impl Layout {
             border: niri_config::Border {
                 off: false,
                 width: FloatOrInt(4.),
-                active_color: Color::new(255, 163, 72, 255),
-                inactive_color: Color::new(50, 50, 50, 255),
+                active_color: Color::from_rgba8_unpremul(255, 163, 72, 255),
+                inactive_color: Color::from_rgba8_unpremul(50, 50, 50, 255),
                 active_gradient: None,
                 inactive_gradient: None,
             },