Tunnel paths should receive different treatment

[nvkelso]

At the Opera metro station in Paris we have multiple underground paths at multiple levels (shows up in different sort_rank values), wow!

Let's give the underground paths a similar treatment to underground roads, and maybe a little different "transparency" (but solid color) depending on the sort_rank?

• https://tangrams.github.io/bubble-wrap/#19/48.87068/2.33195