Add IndexedStackColorMapper for ImageStack glyph

[bryevdv]

Previous PR: #13316

The idea is to be able to display a colormap of just a single "slice" in an image stack as-is, so that e.g. a slider could scrub through the stack with a simple js_link

slider = Slider(start=0, end=nstack-1, value=0, step=1, title="Index")
slider.js_link("value", color_mapper, "index")