Added photon tracing.

sphere_area_light.cpp

@@ -1,7 +1,8 @@
 #include "sphere_area_light.hpp"
 
-void SphereAreaLight::sample_at_surface(vec3 point) {
+vec3 SphereAreaLight::sample_at_surface() {
   Sphere * s = static_cast<Sphere *>(m_figure);
   m_last_sample = m_figure->sample_at_surface();
   m_n_at_last_sample = normalize(vec3((m_last_sample - s->m_center) / s->m_radius));
+  return m_last_sample;
 }