Add two predefined shapes to renderer.
Use quadtree when detecting features in clusters.
Make clustering distance a function (so it is dynamic).
Tiny optimization: do not cast to Float if explicitly asked.

This could improve perfomance when clustering via QuadTree which
extensively uses Bounds.contains() function; in previous version of
Bounds class ~20% of time was spent in OpenLayes.Util.toFloat - and
most of these calls were initiated by QuadTree.retrieveBBox().