Adjust PAL timings

[jfroco]

Fix issue reported here: #118 by @EvoluZion3.

When Video Standard = PAL, the core freezes after ~68 seconds.

Quick Menu -> Core Options -> Video -> Video Standard = PAL

According to upstream emulator ( atari800/atari800@b4bdcee )

The correct values are calculated here: https://github.com/atari800/atari800/blob/cb2bc675e79aaafbeb3494f8b8cfb508c390339e/src/libatari800/sound.c#L53-L58

	double refresh_rate;
	double samples_per_video_frame;

	refresh_rate = Atari800_tv_mode == Atari800_TV_PAL ? Atari800_FPS_PAL : Atari800_FPS_NTSC;
	samples_per_video_frame = setup->freq / refresh_rate;
	setup->buffer_frames = (int)(ceil(samples_per_video_frame));

So, the core now uses these values:

PAL: (int)(ceil(44100 / 49.8607597)) = 885 (it was 883)
NTSC: (int)(ceil(44100 / 59.9227437)) = 736