Combined highway, railway, and aeroway

[pnorman]

Ways with both highway tags and railway tags have been a long-known issue with some styles. CartoCSS-based style had to combine railway and highway into one column to avoid suboptimal performance with multiple columns. Maplibre doesn't have those issues, but we're still combining. This is reasonable, as it makes it much easier to write the styles, but leads to problems with features combining highway, railway, and aeroway.

Using the example from earlier, it's not clear what the attributes of highway=service railway=tram should be.

For my implementation I'm going to treat the layer as a highway, railway, and aeroway layer combined into one. This will mean the same object can appear multiple times if it has both tags, which lends itself to easy style writing.