Fix camera initialization with explicit vup vector

Resolves #523

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -2669,7 +2669,7 @@
 This allows us to change the viewpoint:
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
-    camera cam(point3(-2,2,1), point3(0,0,-1), vup, 90, aspect_ratio);
+    camera cam(point3(-2,2,1), point3(0,0,-1), vec3(0,1,0), 90, aspect_ratio);
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     [Listing [scene-free-view]: <kbd>[main.cc]</kbd> Scene with alternate viewpoint]