MVT rendering issues

[jng]

• We need to apply a buffer on the MVT tile region so that client applications can properly stitch together features that cross tile boundaries
• Multi-polygons that cross tile boundaries are not being rendered Multi-geometries are not supported in the vector tile spec (see: ​4.3.4 Geometry Types), so such multi-geometries should be exploded into its constituent geometries
• Geometries should be simplified at higher levels