Added compute_face_normal function for glm vectors

We have two of these functions now, one for glm::vec3 and one for cv::Vec3f. At some point, this needs clean-up!

include/eos/render/utils.hpp

@@ -99,6 +99,24 @@ cv::Vec3f calculate_face_normal(const cv::Vec3f& v0, const cv::Vec3f& v1, const
 	return n;
 };
 
+/**
+ * Computes the normal of a face (or triangle), i.e. the
+ * per-face normal. Return normal will be normalised.
+ * Assumes the triangle is given in CCW order, i.e. vertices
+ * in counterclockwise order on the screen are front-facing.
+ *
+ * @param[in] v0 First vertex.
+ * @param[in] v1 Second vertex.
+ * @param[in] v2 Third vertex.
+ * @return The unit-length normal of the given triangle.
+ */
+glm::vec3 compute_face_normal(const glm::vec3& v0, const glm::vec3& v1, const glm::vec3& v2)
+{
+	glm::vec3 n = glm::cross(v1 - v0, v2 - v0); // v0-to-v1 x v0-to-v2
+	n = glm::normalize(n);
+	return n;
+};
+
 /**
  * Draws the texture coordinates (uv-coords) of the given mesh
  * into an image by looping over the triangles and drawing each