2.78.0

• Fix potential infinite loops in as-needed dropping and coalescing.
  When the threshold cannot be increased, it is now an error, rather than
  falling back to trying to lower the detail.
• Cleaning of complex polygon geometries now happens in stages
  to avoid performance problems when there are very large numbers
  of vertices.
• Label point generation happens earlier in tiling, to avoid doing slow
  operations on polygons that will not be retained anyway.