What the fork?

The way that I intended this to work is that the front-end should create a thread which calls the M64CMD_EXECUTE command to start the emulator running and returns when finished. The main thread is free to continue accepting user input and making calls into the core as necessary. I would recommend using the SDL threading functions since everything else uses SDL but it shouldn't be an absolute requirement.

There are probably lots of threading bugs lurking since this hasn't been thoroughly tested. However the underlying code works exactly the same as it used to with the old GUIs (a thread was also created in the old architecture) and none of that was formally thread-safe but it mostly worked, so it should still mostly work. As we find bugs I'll add mutexes as necessary to make it more bulletproof.