encoding/mvt: use protoscan for unmarshalling

[paulmach]

Vector Tiles have an encoding for the geometry. This causes an intermediate state were the protobuf is unpacked into a []uint32 and then converted into the orb.Geometry types. This intermediate step can be removed by using a low lever decoder like protoscan

The behavior is the same, but the performance is substantially better:

benchmark                     old ns/op     new ns/op     delta
BenchmarkUnmarshal-12         367590        246198        -33.02%

benchmark                     old allocs     new allocs     delta
BenchmarkUnmarshal-12         6629           2476           -62.65%

benchmark                     old bytes     new bytes     delta
BenchmarkUnmarshal-12         372954        211681        -43.24%