This PR does a couple of things:

• We're currently carrying around two versions of the WWT mode, one which is reactive and one which isn't. This is obviously less than ideal, and we don't actually need the non-reactive version of the mode - WWT knows which mode it's in, and we can find out which one by looking at the type of the background imageset. Thus, this removes the non-reactive mode and queries the background imageset type when we need it outside of the component (only in one place).
• This aims to resolve Handling of Sun marker in RadWave #331 by using a square to represent the Sun. I was originally planning on doing this via an annotation, but that didn't work due to the way that annotations are filled. I won't get into all the details here, but the short version is that triangulation used for the fill is done by adding the center of the annotation as a "vertex" and doing a fan triangulation from there. This is fine in 2D, but doesn't work so well for our 3D purposes. While that's something I can look at upstream (I don't see any reason why we couldn't use e.g. a Delaunay triangulation instead), it won't be trivial. So for this mini, what I've done is hacked the engine to let us use a solid square of the given color for a spreadsheet layer, rather than drawing a square annotation. This does have the disadvantage that it won't rotate with the camera in 3D. Any thoughts are welcome.