Add linear_gradient() helper

lights/src/rgb.rs

@@ -60,3 +60,7 @@ pub const fn gray(gray: u8) -> Rgb {
 pub fn blend(a: Rgb, b: Rgb, mix: Rgb) -> Rgb {
     (a * Rgb(255 - mix.0, 255 - mix.1, 255 - mix.2)) + (b * mix)
 }
+
+pub fn linear_gradient(from: Rgb, to: Rgb, steps: usize) -> impl Iterator<Item = Rgb> + Clone {
+    (0..steps).map(move |x| blend(from, to, gray((x * 255 / steps) as u8)))
+}