add some some color-manipulation tools

lights/src/rgb.rs

@@ -61,6 +61,12 @@ 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)))
+#[inline]
+pub fn blend_steps(from: Rgb, to: Rgb, steps: usize, x: usize) -> Rgb {
+    blend(from, to, gray((x * 255 / steps) as u8))
+}
+
+#[inline]
+pub fn linear_gradient(from: Rgb, to: Rgb, steps: usize) -> impl Iterator<Item = Rgb> + DoubleEndedIterator + Clone {
+    (0..steps).map(move |x| blend_steps(from, to, steps, x))
 }