Some MaxYMiser songs miss beats or crash

[frno7]

MaxYMiser seems to miss beats occasionally, without crashing as it did in #51.

gwEm/Warp_Intro.sndh grinds to a halt at 00:26, see #51 (comment).

[frno7]

I suspect that the DMA sound activity signal might be inverted. Confer figure 15 in the GST Shifter section. Maybe related?

psgplay/lib/atari/mfp.c

Lines 163 to 165 in 56a44b8

	cf68901.port.tai(&cf68901, clk, level); /* FIXME: !level */
	cf68901.port.wr_gpip(&cf68901, clk,
	GPIP_MONITOR_DETECT, level ^ mono_monitor_detect()); /* FIXME: !level */