Markers allow-overlap=true by default at Mapnik 3.x

[springmeyer]

Now that markers support so many cool features, it is much more common to want to allow overlap, and there are now serious performance issues when rendering 1000s or millions of points with allow-overlap=true - the quad_tree takes ages and bloats in memory.

I think we should switch the default at Mapnik 3.x.

[lightmare]

Hasn't happened, which I think is good. I prefer when boolean params consistently default to false. Changing this default would only create issues without solving any.