tuples: implement EPSILON aware PartialEq for Color.

Mark Color::new const so it can be used in const expressions (like
predefining colors).

rtchallenge/src/tuples.rs

@@ -158,17 +158,24 @@ pub fn cross(a: Tuple, b: Tuple) -> Tuple {
     )
 }
 
-#[derive(Copy, Clone, Debug, PartialEq)]
+#[derive(Copy, Clone, Debug)]
 pub struct Color {
     pub red: f32,
     pub green: f32,
     pub blue: f32,
 }
 impl Color {
-    pub fn new(red: f32, green: f32, blue: f32) -> Color {
+    pub const fn new(red: f32, green: f32, blue: f32) -> Color {
         Color { red, green, blue }
     }
 }
+impl PartialEq for Color {
+    fn eq(&self, rhs: &Color) -> bool {
+        ((self.red - rhs.red).abs() < EPSILON)
+            && ((self.green - rhs.green).abs() < EPSILON)
+            && ((self.blue - rhs.blue).abs() < EPSILON)
+    }
+}
 impl Add for Color {
     type Output = Self;
     fn add(self, other: Self) -> Self {