Have WMS meta tiler fall back on GWC where the request structure allows for it

The GeoServer meta tiler is there to provide better labelling for tiled maps. It's general in that it does not assume a specific grid layout, but the cache is in memory and short lived.
If the request is compatible with the GWC cache for that layer (grid location, zoom level, format, filtering options and whatnot) the meta tiler should redirect the request straight to GWC.
The layer preview should also try to align the grid with the GWC one to make the fall back possible, at least for layers in 4326 or 900913 (and for the native SRS if GWC ever learns to cache it as well)