core: return to frontend after certain number of CPU cycles

[deltabeard]

Currently, Peanut-GB uses gb_run_frame() to emulate the Game Boy and returns after a frame is drawn. If the LCD is set to off, then this function would not return until the game turns the LCD back on again. On a real Game Boy, this would result in a delay (typically small). But Peanut-GB skips this delay.

Peanut-GB should return to the frontend after a certain number of CPU cycles (eg. the number of cycles that it would take for a frame to be drawn).