Implement avoid-edges in PointSymbolizer

[kunitoki]

I miss the ability to avoid edges in point symbolizer (i could use a ShildSymbolizer instead but that doesn't accept empty text), as my markers sometimes get clipped at tile edges resulting in a bad generated output.

[springmeyer]

interesting, never realized point symbolizer does not have avoid-edges.

ShieldSymbolizer does have a no-text parameter, although I've not personally tested using it.