DrawingBuffer access for vector drawing lib

[wilbert]

It would be great if DrawingBuffer(), DrawingBufferPitch() and DrawingBufferPixelFormat() could be used within a StartVectorDrawing() / StopVectorDrawing() block.
I understand that this would not be possible for all outputs but for ImageVectorOutput() and CanvasVectorOutput() it should be possible I guess.

[chi]

+1 (if possible)