Review metatiling story

[arneke]

Initially metatiles were clipped to the bounds of the layer, so there would be no WMS request past the advertised bounds. Currently the full metatiles are requested anyway, but requests from clients are chopped strictly to the closest tile within the bounds.

There is code in metatile to crop the metatiles, but this doesn't really seem to get used. This generally doesn't seem to be a problem, but labels may float outside the bounds.