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).

[deltabeard]

When checking whether the LCD is switched on/off using if(!(gb->hram_io[IO_LCDC] & LCDC_ENABLE)), an internal variable should be incremented to keep track of the duration of time that the LCD is switched off (including the number of LCD ticks before it was switched off before HBlank). When this internal variable reaches the same amount of time as a single frame, then Peanut-GB should set gb->gb_frame and return as if a new frame has been drawn.

This internal variable should not be cleared when the LCD is switched on, since the time the LCD is switched off might be less than a frame. In this case, this delay should be propagated until the "frame skip" occurs when the LCD has been switched off for long enough to be equal to a single frame.