Add support for variable opacity

[astrofrog]

One thing that Matplotlib images support that would be good to add here is support for colormaps that have variable opacity - for instance when showing a density map of points, it can be useful to have a colormap that is all the same shade of e.g. glue but using opacity variations for the intensity - this allows multiple different density maps to be shown at the same time.

We could add this here by adding a new scale opacity that could be set to a LinearScale, then update the JS code and shader so that it takes this into account.

[maartenbreddels]

We can also have an array with values, and we upload that to the shader as texture.