Adjust maxzoom to avoid loading empty tiles

[zabop]

At zoom level 19, the service mentioned in this source does return useful tiles, for example https://apps.kontur.io/raster-tiler/oam/mosaic/19/315735/264021.png:

However, at zoom level 20, the tiles returned are empty. https://apps.kontur.io/raster-tiler/oam/mosaic/20/631470/528042.png:

I hope limiting maxzoom to 19 will avoid situations when seemingly everything is ok, but the iD canvas is empty:

[arch0345]

Hmm, it looks like in some places tiles go up to zoom 21. Ideally these tiles should return 404s instead of black tiles, not sure if this issue is caused by Kontour's mosaic process or the original upload to OAM