Consider using VisualizerMapping to allow rasterization of visualizers

[bruno-f-cruz]

It is not uncommon that users ask how to save visualizers to a file. While saving static visualizers is supported it would be interesting to think of ways to expose this behavior in a more generic way.

One syntax that could be interesting is using 'VisualizerMapping' to connect any operator to a 'GetPixels' operator that dumps the OpenGL window to a IplImage. Several details must be considered here (like decoupling sampling rate, ensure users understand that this operation might destabilize the rest of the workflow etc...).