Convert depth peeling render pass to the OpenGL2 backend.
Also add a new class that encapsulates the basic rendering
steps that VTK uses.

Modify the renderer to use the depth peeling pass instead of
having two copies of the code lying around. Removed the depth
peeling code from Mapper2D as it is no longer needed now that we
have CopyToFrameBuffer working. That is a lighter weight way to
get a quad up on the screen. Plus that keeps the depth peeling
code more localized as it should be. Clean up a lot of depth
peeling code that was in the renderer and now there are cleaner
ways of doing it (passing information keys).

Change-Id: I8d0a53bb25920bd30eade49ed12126b9b44be358