Run processor step in another thread

[CaffeineViking]

I didn't realize this back then, but the reason the emulation can sometimes slow down to a halt is simply because we're running the processor.step() in tandem with the display (which has v-sync). The simple solution is to just run the processor.step() in a new thread and keep the display and processor timelines separate. Once the processor has a frame to display, it should just poke a flag and the renderer should grab the last display buffer from memory.