Support fade-in/out of image overlays in tours

[pkgw]

@astrodavid10 reports that on the Windows client, overlay images can fade in and out, but that this doesn't work on the webclient.

@astrodavid10, do you happen to have a link to an example tour demonstrating this effect?

Note that the motivation for this is mainly to figure out a way to be able to get reliable text overlays working in both Windows and web clients. The differences in text layout between Windows and web are really substantial, and they make it really hard to use text overlays as intended. Using overlay images would be less-than-ideal, but probably way less work than tackling text overall.

[astrojonathan]

Probably deserves a bit of work to match rendering between the two. The Web Client went thru HTML5 Text before going to WebGL so it does not work the same way, but maybe we can match it up now that they both are pure (ish) 3d environments and we don't have to support backwards compatibility with HTML5 rendering anymore.

[pkgw]

I was able to make progress on this with a fix to the webclient (WorldWideTelescope/wwt-web-client#333): its colorpicker wasn't saving colors correctly. Now that it does, you can create an element in a slide on the webclient, turn its Animate attribute to True, and set start and end colors with differing opacities. A little limited in terms of what you can accomplish, but the effect renders correctly in the webclient, so it's a start.