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
• Certain geometries near tile boundaries are not rendering at certain levels. Examples include:
  • China from the ​NaturalEarth countries SHP file

[jng]

In 9834:

Merged revision(s) 9828-9833 from sandbox/jng/mvt_render_fixes:

#2836: Re-instate clipping of MVT tile geometries at the stylizer level by having MVTRenderer::RequiresClipping?() return true and having MVTRenderer::GetBounds?() return a buffered extents (where buffer is the default of 80 screen pixels). We still do not have the undesired tile border artifact, which tells us that we're buffering by the correct amount. Having said that, this commit also adds a flag to render tile borders for visual debugging purposes. If you see tile border artifacts, it should now be due to this new flag being true.

#2836: Remove geometry clipping code from the ProcessXXX renderer methods. This eliminates the tile border artifact we are experiencing in client applications like OpenLayers/QGIS. If we are meant to clip, we should probably do it at the Stylizer level by having our MVTRenderer return true for RequiresClipping?() and find a way to let the renderer customize the clip extents.
........

[jng]

Re-opening as this was erroneously closed as a result of the merge commit