Add sky Plane.

bheisler/src/lib.rs

@@ -58,6 +58,23 @@ fn test_can_render_scene() {
                     blue: 0.3,
                 },
             }),
+            Element::Plane(Plane {
+                origin: Point {
+                    x: 0.0,
+                    y: 0.0,
+                    z: -20.0,
+                },
+                normal: Vector3 {
+                    x: 0.0,
+                    y: 0.0,
+                    z: -1.0,
+                },
+                color: Color {
+                    red: 0.2,
+                    green: 0.3,
+                    blue: 1.0,
+                },
+            }),
             Element::Sphere(Sphere {
                 center: Point {
                     x: 2.0,