Simplify the OpenGL render pocess a bit

VTK will always render into a RenderFramebuffer If Swapbuffers is on it will also blit into DisplayFramebuffers which can then be used to blit to and draw destination. The render framebuffer may be multisampled. The display framebuffers are not so they could be considered resolve buffers.

Helper functions for GUI integration have been added to blt into the default hardware buffers or to blit into the current draw destination.

Add a ivar to control frame blit behavior. Defaults to blit to hardware, but also has blitToCurrent and NoBlit options.