Cache all drawn polydata instead of just the last one

Because the same vtkOpenGLContextDevice2D instance is responsible for all calls to vtkContext2D::DrawPolyData, it makes sense to cache them all instead of just the last one. This prevents large polydata which have not been modified from having these buffers rebuilt every time Paint is called, drastically improving interaction performance. This is especially important because just moving the mouse across an interactive plot area can trigger many Paint calls.