library: light: add Light trait definition

src/light/mod.rs

@@ -0,0 +1,8 @@
+use super::core::LinearColor;
+use super::Point;
+
+/// Represent a light in the scene being rendered.
+pub trait Light: std::fmt::Debug {
+    /// Get the illumination of that light on that point.
+    fn illumination(&self, point: &Point) -> LinearColor;
+}