colors in transfer function editor

## Use Case

I want to be able to set color gradients based on intensities, using the transfer function editor. In this way, I can view a single channel data source with colors that change according to intensity.

## Solution

Make editor taller. Each control point gets a color by double clicking it. Draw color gradient across the widget area, or color the control points. Expand internal LUT implementaton to handle rgba color and the pathtrace shader to sample colors.