This changes the way texture requests are handled, the AssetManager
RequestImage method signatures have been changed and are now expecting
the callback with the request.
Progressive requests for textures are supported for rendering viewers
Tuning parameters have been moved to Settings
git-svn-id: http://libopenmetaverse.googlecode.com/svn/libopenmetaverse/trunk@2699 52acb1d6-8a22-11de-b505-999d5b087335