With this you can pick an array component and draw that directly
on the screen. Afterward, if you know the min and max, you read pixel
values back and get a hold of the value at each pixel.

TODO:
* Lots of type conversion here, and in the end only using 24 bits
of resolution. When and if we need more accuracy, flesh out
the extensible conversion operator part.
* Surface selection is based on the same concept (ident buffer)
but that was written before VTK has extensible rendering (painters
and passes). Eventually selection should be reimplemented to use
this.
* Would be great to demonstrate render into background followed by
mouse overs.

Change-Id: I44608759278c37c8250f957d596f3f4149382bbb