Scene picker is missing a get depth function

[rchristie]

Application: need mouse coordinates AND depth to transform into world coordinates; from these can perform find mesh location to constrain to surfaces.

Alternatively, a convenience function for getting nearest mesh location could be offered.

Note there have also been requests to get nearest location on surface graphics; this would actually help perform a better find mesh location since it should get the intersection between the ray and the polygon; getting the depth from the picking buffer gives a closer location in general since the picking box is finite size.