Picking operation is not working in Emscripten + vtkSDL2OpenGLRenderWindow

I am trying several operations for newly updated VTK with emscripten rendering.

I found that [Picking Example](https://vtk.org/Wiki/VTK/Examples/Cxx/Interaction/Picking) seems not working for now.

Getting 2D coordinates works fine, but `vtkPropPicker->Pick(...)` returns warning and wrong results.

```

virtual void OnLeftButtonDown() override
{
int* clickPos = this->GetInteractor()->GetEventPosition();

// Pick from this location.
vtkSmartPointer picker = vtkSmartPointer::New();
picker->Pick(clickPos[0], clickPos[1], 0, this->GetDefaultRenderer());

double* pos = picker->GetPickPosition();
std::cout << "Pick position (world coordinates) is: " << pos[0] << " " << pos[1] << " " << pos[2] << std::endl;
std::cout << "Picked actor: " << picker->GetActor() << std::endl;
```

The picked position in world coordinates is always 0 0 0, and the browser shows warning saying :

```
WebGL: INVALID_ENUM: readPixels: invalid format

  | _glReadPixels | @ | Picking.js:11102 -- | -- | -- | --
  | vtkOpenGLRenderWindow::GetZbufferData(int, int, int, int, float*) | @ | 018741d6:1
  | vtkOpenGLRenderWindow::GetZbufferData(int, int, int, int) | @ | 018741d6:1
  | vtkHardwareSelector::BuildPropHitList(unsigned char*) | @ | 018741d6:1
  | vtkHardwareSelector::ProcessPixelBuffers() | @ | 018741d6:1
  | vtkHardwareSelector::Render(vtkRenderer*, vtkProp**, int) | @ | 018741d6:1
  | vtkOpenGLRenderer::UpdateGeometry(vtkFrameBufferObjectBase*) | @ | 018741d6:1
  | vtkOpenGLRenderer::DeviceRender() | @ | 018741d6:1
  | vtkRenderer::Render() | @ | 018741d6:1
  | vtkRendererCollection::Render() | @ | 018741d6:1
  | vtkRenderWindow::DoStereoRender() | @ | 018741d6:1
  | vtkRenderWindow::Render() | @ | 018741d6:1
  | vtkOpenGLRenderWindow::Render() | @ | 018741d6:1
  | vtkHardwareSelector::CaptureBuffers() | @ | 018741d6:1
  | vtkHardwareSelector::Select() | @ | 018741d6:1
  | vtkRenderer::PickProp(double, double, double, double) | @ | 018741d6:1
  | vtkRenderer::PickProp(double, double) | @ | 018741d6:1
  | vtkViewport::PickPropFrom(double, double, vtkPropCollection*) | @ | 018741d6:1
  | vtkPropPicker::PickProp(double, double, vtkRenderer*) | @ | 018741d6:1
  | vtkPropPicker::Pick(double, double, double, vtkRenderer*) | @ | 018741d6:1
  | MouseInteractorStyle2::OnLeftButtonDown() | @ | 018741d6:1

```
```
[.WebGL-000001CEB1519F40] GL_INVALID_OPERATION: Invalid format and type combination.
```