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.