The color transfer function control points being sent from the
client's color editor to the discretized color transfer function
proxy (and thus eventually to the server) were having their
colors quantized on the client before being issued to the proxy.
This caused problems when switching to/from indexed mode because
the `vtkLookupTable` holding the quantized color is very small
in indexed lookup mode.

So, don't quantize where we have no need to; use the control point
values directly.

Change-Id: I8904f0d5e12495f9c6765531bd5a15574fbb2978