Keep (default or custom) tiles URL around
This fixes an error introduced by
b0d2fbc5 where the PMTiles object was
initialized correctly, but the MLGL Map was given pmtiles://undefined
as its vector data source.
This fixes an error introduced by
b0d2fbc5 where the PMTiles object was
initialized correctly, but the MLGL Map was given pmtiles://undefined
as its vector data source.