VecTiles: constrain output geometry to the same type as the input
N.B.: adding this because of cases where line + tile bounding box intersections return points (instead of lines), which can be difficult to handle client-side; cleaner to make the assumption that a query for lines will always return lines
it's OK to mix 'multi' geometry of the same type (e.g. MULTIPOLYGON source geoms outputting to POLYGON)
Merge pull request #11 from mapzen/vectiles-constrain-geom-type

Vectiles constrain geom type