Adds an event and a method so that handling of the CachedTexture
packet can be pulled out of LLClientView and moved to
AvatarFactory. The first pass at reusing textures (turned off by
default) is included. When reusing textures, if the baked textures
from a previous login are still in the asset service (which generally
means that they are in the simulator's cache) then the avatar will not
need to rebake. This is both a performance improvement (specifically
that an avatars baked textures do not need to be sent to other users
who have the old textures cached) and a resource improvement (don't
have to deal with duplicate bakes in the asset service cache).