Geometry Nodes: Increase parallelism in Geometry Proximity node

The default grain size of 10000 for multi function execution was much
too large for the fairly expensive BVH traversal. In the file from
#143455, reducing te grain size to 512 increase the playback FPS from
8 to 20.

source/blender/nodes/geometry/nodes/node_geo_proximity.cc

@@ -247,6 +247,13 @@ class ProximityFunction : public mf::MultiFunction {
       }
     });
   }
+
+  ExecutionHints get_execution_hints() const override
+  {
+    ExecutionHints hints;
+    hints.min_grain_size = 512;
+    return hints;
+  }
 };
 
 static void node_geo_exec(GeoNodeExecParams params)