message: Print FPS in volumetric path tracing example
diff: #include <common/manip/pan_manipulator.h>
  #include <common/manip/translate_manipulator.h>
  #include <common/manip/zoom_manipulator.h>
  +#include <common/timer.h>
  #include <common/viewer_glut.h>
  using namespace visionaray;
  @@ -188,6 +189,9 @@ struct renderer : viewer_type
  pinhole_camera cam;
  cpu_buffer_rt<PF_RGBA8, PF_UNSPECIFIED, PF_RGBA32F> host_rt;
  tiled_sched<host_ray_type> host_sched;
  + frame_counter counter;
  + double last_frame_time = 0.0;
  + bool print_fps = true;
  unsigned frame_num = 0;
  @@ -382,6 +386,13 @@ void renderer::on_display()
  return result;
  }, sparams);
  + last_frame_time = counter.register_frame();
  +
  + if (print_fps)
  + {
  + std::cout << "FPS: " << last_frame_time << '\r';
  + std::cout << std::flush;
  + }
  // display the rendered image
  