Rename _game to _steamGame in ScummSteamFile

This avoids confusion with the global _game variable, as _steamGame
has slightly different semantics: it contains the original game
platform (Windows or Mac), so that the correct game executable is
used. However, the platform in _game is always set to DOS, since all
the Steam versions use the same DOS data files
SCUMM: Fix comment
SCUMM: Some fixes to the Audio CD code for the Steam version of Loom

- Reset the bit shift values when seeking
- Use the same DisposeAfterUse flag in CDDAStream as the other stream types
- Fix formatting