message: Add "if" when compiling subpass material.
  This avoids error spew on some platforms such as Chrome + WebGL 2.0
diff: @@ -105,6 +105,8 @@ PostProcessManager::PostProcessManager(FEngine& engine) noexcept : mEngine(engin
  }
  void PostProcessManager::init() noexcept {
  + DriverApi& driver = mEngine.getDriverApi();
  +
  // TODO: load materials lazily as to reduce start-up time and memory usage
  mSSAO = PostProcessMaterial(mEngine, MATERIALS_SAO_DATA, MATERIALS_SAO_SIZE);
  mMipmapDepth = PostProcessMaterial(mEngine, MATERIALS_MIPMAPDEPTH_DATA, MATERIALS_MIPMAPDEPTH_SIZE);
  @@ -116,7 +118,9 @@ void PostProcessManager::init() noexcept {
  mBlit[1] = PostProcessMaterial(mEngine, MATERIALS_BLITMEDIUM_DATA, MATERIALS_BLITMEDIUM_SIZE);
  mBlit[2] = PostProcessMaterial(mEngine, MATERIALS_BLITHIGH_DATA, MATERIALS_BLITHIGH_SIZE);
  mTonemapping = PostProcessMaterial(mEngine, MATERIALS_TONEMAPPING_DATA, MATERIALS_TONEMAPPING_SIZE);
  + if (driver.isFrameBufferFetchSupported()) {
  mTonemappingWithSubpass = PostProcessMaterial(mEngine, MATERIALS_TONEMAPPINGWITHSUBPASS_DATA, MATERIALS_TONEMAPPINGWITHSUBPASS_SIZE);
  + }
  mFxaa = PostProcessMaterial(mEngine, MATERIALS_FXAA_DATA, MATERIALS_FXAA_SIZE);
  mDoFBlur = PostProcessMaterial(mEngine, MATERIALS_DOFBLUR_DATA, MATERIALS_DOFBLUR_SIZE);
  mDoF = PostProcessMaterial(mEngine, MATERIALS_DOF_DATA, MATERIALS_DOF_SIZE);
  @@ -129,7 +133,6 @@ void PostProcessManager::init() noexcept {
  // The total number of samples needed over the two passes is 18.
  mSeparableGaussianBlurKernelStorageSize = mSeparableGaussianBlur.getMaterial()->reflect("kernel")->size;
  - DriverApi& driver = mEngine.getDriverApi();
  mDummyOneTexture = driver.createTexture(SamplerType::SAMPLER_2D, 1,
  TextureFormat::RGBA8, 0, 1, 1, 1, TextureUsage::DEFAULT);
  