[sldev] Texture Cache in Simulator?

[Ed Anuff]

Maybe this is already in place, but, since textures are immutable, does it
make sense for the sim to maintain a copy of some number of textures in use
in a region?  It would make it easier to send these en masse on region entry
to make things visible more quickly.  Haven't looked at the code in this
regards, so as I said, maybe this is already the way it's done, but it seems
like textures are pulled on-demand rather than pushed pre-emtively.