Color scale in Rendering Settings dialog does not update

[Peter Ellis]

The color scale image in the Rendering Settings dialog does not update when the user changes the value in the Color Scheme drop-down menu from "Brightness Scale" to "Hue Scale". The image should change from gray scale to "rainbow" when the value is changed.

[Peter Ellis]

This is fixed on Windows with commit e89a987, but still broken on Mac due to a bigger problem (see Issue #35).