add a Check method to flotsamAssetCache, so to check if a asset is in

cache without actually loading it. Make use limited use of it in
avatarfactory textures check. Also on llclientview
HandleAgentTextureCached that now should work. Other asset cache modules
for now will return false, so are broken. baked textures logic
still unchanged. *UNTESTED*

OpenSim/Framework/IImprovedAssetCache.cs

@@ -33,6 +33,7 @@ namespace OpenSim.Framework
     {
         void Cache(AssetBase asset);
         AssetBase Get(string id);
+        bool Check(string id);
         void Expire(string id);
         void Clear();
     }