Nice-to-have: nice embed support for "mid-ground" imagery

[pkgw]

@astrodavid10 reports that he often wants to set up an embed with "mid-ground" imagery: say DSS for the all-sky background, then a nicer large mosaic around an object of interest, then an even more detailed image of the object itself.

The engine can certainly support this, but the embed doesn't provide a convenient way to set it up. I can think of a few ways to approach it:

• Add some special processing of WTML files to auto-create multiple layers from the imagery when requested
• Add support for loading WWTL files (which should be 99% there already for the tour support)
• Add some special support for one-slide tour files

At the moment I think that the last of these is probably the best, since the tour file already encodes things like the camera position, which a WWTL file won't. We could maybe wire in a mode where a tour file is loaded up and set up as per its first stop, but then instead of playing back, we just exit the tour mode while preserving all of the layers and camera and whatnot. The engine has a lot of infrastructure to make sure that state set up by tours is ephemeral, but we could probably find a way to permanent-ify it.

CC @Carifio24