config: Premultiply alpha in Color when converting to f32

Smithay wants premultiplied alpha.

niri-config/src/lib.rs

@@ -348,7 +348,8 @@ impl Color {
 
 impl From<Color> for [f32; 4] {
     fn from(c: Color) -> Self {
-        [c.r, c.g, c.b, c.a].map(|x| x as f32 / 255.)
+        let [r, g, b, a] = [c.r, c.g, c.b, c.a].map(|x| x as f32 / 255.);
+        [r * a, g * a, b * a, a]
     }
 }