Access relation members from `relation_scan_function`

[ginnyTheCat]

I was trying to render public_transport=stop_area to show subway station areas using the relation_function and it works great.

However some bus stations have stop_area relations as well, which are the "artifacts" right of J2 and above J5 for example. The relation itself does however not know what type of transit it represents.

In overpass I filtered out all relations without a railway=station member to only get stop_areas that aren't bus stops and that got rid of the artifacts. Afaik there is no way to access tags of relation members like that in tilemaker.

Inverting this and rendering from the way_function would not work as parts of stations are not railway=station, but still part of the relation. In those way_functions I wouldn't know whether the relation has any railway stations or is a bus station. Also I would need to accept all ways as these areas can have arbitrary tags, they just need to be in the stop_area relation.

The best option to solve this would probably be something similar to the NextMember functions proposed in #632. Maybe there is some other way to solve this that I'm just missing though.