Added presentations from EVI 2017

EVI - 2017/EVI 21/EVIray/sphere_area_light.cpp

@@ -0,0 +1,8 @@
+#include "sphere_area_light.hpp"
+
+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;
+}