Fix QVTKOpenGLWidget rendering with disabled interactor
In QVTKOpenGLWidget, paintGL() assumes that renderVTK() always renders something into the framebuffer. However, rendering doesn't occur when the vtkRenderWindowInteractor EnableRender flag is disabled. In this case, QVTKOpenGLWidget can display the contents of an uninitialized framebuffer.
This commit clears the VTK rendering framebuffer upon creation. This ensures that paintGL() doesn't blit an uninitialized framebuffer when rendering doesn't occur in renderVTK().
One example when the vtkRenderWindowInteractor EnableRender flag would be disabled is when an application enforces a specified framerate by rendering only when a timer fires, not in response to interaction events.