Extend BCFViewPointsPlugin to save & load object colors & opacities

[Amoki]

Currently, the BCF plugin doesn't handle the "color" part.
I'll submit a PR the handle them in setViewpoint but I have some trouble implementing getViewpoint.

We should define "colors" only for objects that have seen their color changed in the current scene. Is there a way to get all the objects with a changed color?

[xeolabs]

@Amoki it should be possible to get the IDs of Entity instances that have had their colorize properties modified like this this:

const colorizedObjectIds = myViewer.scene.colorizedObjectIds;

[Amoki]

Thanks, I could develop the coloring part.
I can't find an equivalent for opacity. I tried to loop on objects and check their opacity but I get all windows and transparent doors. Is there a way to retrieve objects that had an opacity change?

[xeolabs]

Yes looks like I need to add something for opacity - I'll take care of this today or tomorrow: #343

[xeolabs]

@Amoki I just added support that this issue needs for opacity: 37dc831