[Qgis-developer] [WIP] Cluster renderer

[Nyall Dawson]

Announcing WIP regarding a new "cluster" renderer for symbology. The
cluster renderer does the opposite of the existing displacement
renderer and instead collapses nearby points into a single symbol,
much like the cluster markers which are common in web mapping
libraries like Leaflet.

Note that the work is NOT definite yet, but please contact me if you
have work planned involving QgsPointDisplacementRenderer.

Nyall