add pixel read feature

[rogerallen]

From a discussion on the google group:
https://groups.google.com/forum/?fromgroups#!topic/overtone/cVIZKiZRuwk

Circu Virtu asks:

Some new questions pop into my mind:

Is there any way to get pixel data back from the shader into overtone (creating , controlling music from grafic data) ??

That's just a glReadPixels call, so it isn't too difficult to add. The amount of data could be significant, though and I'd like to hear more about how you'd intend to use it.

Good to hear that there is a possible solution. I'm not interested in getting all Pixel data. I'm most interested in just values of single pixels or means of small pixel areas. See them as sensors for brightness and/or color of the image.
The computing power of the shader is amazing. That way one can get data back to overtone to control parameters fro music.
And even better one then can create recursive/iterrative loops like: sound changes images -> images change sound -> sound changes images -> and so on.

Would love to have an easy to use interface to access glPixel.