Support for Ogg files in the sound manager

The sound manager currently only can play wav files. We should add support for Ogg files too. This involves using the data parsing callback functionality in the vorbis library (in contrast to using the file based loading methods). This is because we use the Ogre resource system for loading data, and thus cannot use the lower level FILE operations.

Note that support for ogg files are separate from support for streaming sounds: any ogg sound that's small enough could be static, i.e. loaded into memory at once.

In order for this to be implemented we need to alter the IResourceProvider interface a little to allow for dynamic loading of data (instead of having it load all data up front as it does currently).