WIP: Support mounting a physical CD-ROM drive on Windows

[beholdnec]

At the risk of suffering burnout... here's a PR to support mounting a physical CD-ROM drive on Windows, so you can play real discs in Snow.

TODO:

• Support mounting and opening a data CD
• Support mounting and playing an audio CD
• Support mounting and playing a multi-session CD (at least the audio portion should be playable)
• Add a menu for listing and selecting physical drives
• Handle gracefully if the user ejects the CD

Known issues:

• The emulator will briefly freeze on each access; Snow can't (yet) do anything else while a SCSI read is in progress.
• If you mount an iso via Microsoft Virtual DVD-ROM (i.e. when you double-click the iso file in Explorer), Mac OS will report "This disk is unreadable by this Macintosh." Loading the iso file directly in Snow works fine.

Notes:

• I don't have access to a Linux or Mac machine for testing. This PR will implement this feature for Windows only.

[twvd]

"Allow emulator to continue running while disc accesses are in progress" will probably be pretty involved because you'd need to implement all the delay/busy logic in the SCSI controller, which is a bit of a project of its own. Snow now only responds to any SCSI command immediately. I wouldn't recommend you try to tackle that as part of this PR.