Fix #if MACOS to be #if MONOMAC (fixes bugs with content loading & probably audio)

There were a couple "#if MACOS" directives in TitleOpenContainer.cs and OpenALSoundController.cs, but MACOS isn't a valid define symbol for any platform. The proper symbol to check against is MONOMAC. The attached commit fixes that.

The bug made all content loading on Mac be relative to the current working directory so I suppose it went undetected because few people are changing the current working directory.

It also made the call to alcMacOSXMixerOutputRate in OpenALSoundController never happen on Mac. Not sure what are the implications of that but I guess it's a good thing it's not ignored anymore.