This changes the detail sampling to find height data by using the flood
fill method for polygons that were merged because of a border vertex
removal. This could potentially cause overlapping polygons to receive
the same region IDs which would later make the fast-path sampling in
getHeightData sample the wrong heights.
Also includes optimizations for this path and getHeightData itself.
Previously the seeding was responsible for 25% of the total time spent
in getHeightData in these cases. We now use the direction and prefer
moving directly towards the center of the polygon. This makes the
overhead of seedArrayWithPolyCenter virtually nonexistent (~2%
according to profiling).
Fix#146
