spiderfyShapePositions now receives the full list of childMarkers

[EliteMasterEric]

Done as a third argument to prevent changing the method signature.

One example use case for this is creating a spiderfy in which all markers spider to their original positions, rather than in a circle or spiral.

[EliteMasterEric]

I have discovered that the use case that this change required can be implemented by instead using the original-locations options of https://github.com/adammertel/Leaflet.MarkerCluster.PlacementStrategies.