Fixed bug that was causing spurious NaN pixels.

It was because sometimes the PDF's for a sample could be zero,
leading to a divide by zero.  PDF = 0.0 is now checked for.

src/renderer.rs

@@ -392,6 +392,8 @@ impl LightPath {
                                             self.wavelength,
                                             self.time,
                                             isect) {
+                        // Check if pdf is zero, to avoid NaN's.
+                        if light_pdf > 0.0 {
                             // Calculate and store the light that will be contributed
                             // to the film plane if the light is not in shadow.
                             self.pending_color_addition = {
@@ -419,6 +421,9 @@ impl LightPath {
                             true
                         } else {
                             false
+                        }
+                    } else {
+                        false
                     };
 
                     // Prepare bounce ray
@@ -433,6 +438,8 @@ impl LightPath {
                             material.sample(idata.incoming, idata.nor, (u, v), self.wavelength)
                         };
 
+                        // Check if pdf is zero, to avoid NaN's.
+                        if pdf > 0.0 {
                             // Account for the additional light attenuation from
                             // this bounce
                             self.next_attentuation_fac = filter.e / pdf;
@@ -445,6 +452,9 @@ impl LightPath {
                                                                  false));
 
                             true
+                        } else {
+                            false
+                        }
                     } else {
                         self.next_bounce_ray = None;
                         false