Added time parameter to shader evaluation.

It's not used right now, but in the future I want shaders to be
able to vary over time and have motion blur.  This serves as a
nice little reminder by putting it in the API.

src/shading/mod.rs

@@ -10,7 +10,12 @@ use surface::SurfaceIntersectionData;
 pub trait SurfaceShader: Debug + Sync {
     /// Takes the result of a surface intersection and returns the surface
     /// closure to be evaluated at that intersection point.
-    fn shade(&self, data: &SurfaceIntersectionData, wavelength: f32) -> SurfaceClosureUnion;
+    fn shade(
+        &self,
+        data: &SurfaceIntersectionData,
+        time: f32,
+        wavelength: f32,
+    ) -> SurfaceClosureUnion;
 }
 
 /// Clearly we must eat this brownie before the world ends, lest it
@@ -37,8 +42,13 @@ pub enum SimpleSurfaceShader {
 }
 
 impl SurfaceShader for SimpleSurfaceShader {
-    fn shade(&self, data: &SurfaceIntersectionData, wavelength: f32) -> SurfaceClosureUnion {
+    fn shade(
-        let _ = data; // Silence "unused" compiler warning
+        &self,
+        data: &SurfaceIntersectionData,
+        time: f32,
+        wavelength: f32,
+    ) -> SurfaceClosureUnion {
+        let _ = (data, time); // Silence "unused" compiler warning
 
         match *self {
             SimpleSurfaceShader::Emit { color } => {

src/surface/triangle_mesh.rs

@@ -249,7 +249,11 @@ impl<'a> Surface for TriangleMesh<'a> {
                                 // Fill in intersection data
                                 isects[r.id as usize] = SurfaceIntersection::Hit {
                                     intersection_data: intersection_data,
-                                    closure: shader.shade(&intersection_data, wr.wavelength),
+                                    closure: shader.shade(
+                                        &intersection_data,
+                                        wr.time,
+                                        wr.wavelength,
+                                    ),
                                 };
                                 r.max_t = t;
                             }