message: Missing backend now throws Java exception, not segfault.
  If a client application selects a backend that is not supported by the
  build, Engine::create() now returns null, which allows the Java wrapper
  to throw IllegalStateException. Prevously, we would crash in the native
  layer by hitting a null pointer in Engine::loop().
diff: @@ -114,6 +114,10 @@ FEngine* FEngine::create(Backend backend, Platform* platform, void* sharedGLCont
  instance->mPlatform = platform;
  instance->mOwnPlatform = true;
  }
  + if (platform == nullptr) {
  + slog.e << "Selected backend not supported in this build." << io::endl;
  + return nullptr;
  + }
  instance->mDriver = platform->createDriver(sharedGLContext);
  } else {
  // start the driver thread
  @@ -430,6 +434,11 @@ int FEngine::loop() {
  break;
  }
  slog.d << io::endl;
  + if (mPlatform == nullptr) {
  + slog.e << "Selected backend not supported in this build." << io::endl;
  + mDriverBarrier.latch();
  + return 0;
  + }
  }
  #if FILAMENT_ENABLE_MATDBG
  