message: add help/usage text to ospExamples
diff: // stl
  #include <iostream>
  -#include "example_common.h"
  #include "GLFWOSPRayWindow.h"
  +#include "example_common.h"
  using namespace ospray;
  static std::string rendererType = "pathtracer";
  static std::string builderType = "perlin_noise_volumes";
  +void printHelp()
  +{
  + std::cout <<
  + R"description(
  +usage: ./ospExamples [-h | --help] [[-s | --scene] scene] [[r | --renderer] renderer_type]
  +
  +scenes:
  +
  + boxes
  + cornell_box
  + curves
  + cylinders
  + empty
  + gravity_spheres_volume
  + perlin_noise_volumes
  + random_spheres
  + streamlines
  + subdivision_cube
  + unstructured_volume
  +
  + )description";
  +}
  +
  int main(int argc, const char *argv[])
  {
  initializeOSPRay(argc, argv);
  for (int i = 1; i < argc; ++i) {
  std::string arg = argv[i];
  - if (arg == "-r" || arg == "--renderer")
  + if (arg == "-h" || arg == "--help") {
  + printHelp();
  + return 0;
  + } else if (arg == "-r" || arg == "--renderer") {
  rendererType = argv[++i];
  - if (arg == "-s" || arg == "--scene")
  + } else if (arg == "-s" || arg == "--scene") {
  builderType = argv[++i];
  }
  + }
  {
  auto builder = testing::newBuilder(builderType);
  