Reduce surface tag precision at mid-zooms to 3 values

[nvkelso]

Originally posted by @zerebubuth in #1710 (comment)

In #1710 we stripped all surface properties at mid- zooms. Instead we should re-map surface tags at mid-zooms to still promote more merging, but keep some data fidelity.

It turns out @matkoniecz makes use of those tags in #1710 (comment):

• https://github.com/matkoniecz/Zazolc (personal fork of StreetComplete).
• Style is at https://github.com/matkoniecz/Zazolc/blob/fork/app/src/main/assets/map_theme/fork-style.yaml

The full and raw values would still be available past that zoom (like 13 or 15), so I think these 3 category mappings (paved, compacted, unpaved) are fine at mid-zooms, even for cyclist since they could zoom in and see full details.

For just minor_roads we'd also backfill missing surface tags as paved to promote more merging (rather than leaving it null). But for other road kinds we'd leave null as null.

            # paved
            good_surface: 
                filter: {surface: [asphalt, paved, wood, metal, tartan, metal_grid]}

            # compacted
            medium_surface:
                filter: {surface: [concrete, paving_stones, compacted, sett]}

            # unpaved
            bad_surface:
                filter: {surface: [unpaved, dirt, earth, fine_gravel, grass, grass_paver, gravel,
                ground, mud, pebblestone, salt, sand, woodchips, clay, cobblestone,
                cobblestone:flattened, concrete:lanes, concrete:plates, artificial_turf, decoturf]}
                draw:

[nvkelso]

Verified.