Fix invalid multipoint geometry encoding

[anandthakker]

Multipoint geometries are currently encoded as multiple 1-length MoveTo commands. The vector-tile-js library decodes this fine, but it's incorrect according to the spec:

4.3.4.2. Point Geometry Type
The POINT geometry type encodes a point or multipoint geometry. The geometry command sequence for a point geometry MUST consist of a single MoveTo command with a command count greater than 0.

If the MoveTo command for a POINT geometry has a command count of 1, then the geometry MUST be interpreted as a single point; otherwise the geometry MUST be interpreted as a multipoint geometry, wherein each pair of ParameterIntegers encodes a single point.

https://github.com/mapbox/vector-tile-spec/blob/master/2.1/README.md#4342-point-geometry-type