also fixes 3 bugs:
1) mpeg right channel volume was always using the left channel volume, too
2) too high MusicVolume setting was not clamped to 0..200
3) too high SoundVolume setting was not clamped to 0..200
- Added object interface CThread for OSD threading and synchronization, together with SDL implementation
- Added multi-threading to CModel3 so that separate CPUs (PPC of main board, 68K of sound board and Z80 of drive board) can be run in separate threads if requested to improve performance on multi-core PCs
- Added -multi-threaded command line option (default is to run single-threaded still)