generate
Generates every requested contour level from tile's elevation grid, chained into polylines.
Parameters
tile
an ElevationTile. Unlike Hillshade.shade, this doesn't need cross-tile margin padding — contour geometry only needs it if lines should visually continue past this tile's own edge, which is the caller's choice, not a requirement here.
levels
the elevations (meters) to contour at — see ContourIntervals for the zoom-banded table this is normally called with.