Feature request: Transparent clusters which does not increment the background color index

[spar-eag]

In e.g. AWS architecture diagrams it's quite common to have groups with no background color. It's also quite common to have deeply nested groups (e.g. AWS cloud -> AWS account -> AWS region -> ...).

We can get a similar result by creating nested Clusters, and setting graph_attrs={"bgcolor": "") when creating the Cluster. However, if we then create a new cluster which does have a background color, we end up an unexpected color, and some colors might go entirely unused, since the color index is picked on depth.

It would be nice if we would only consider Clusters which actually display a background color when calculating the depth for the purpose of getting the color index.