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games/opl3: implement real time streaming mode

this pushes all responsibility for timing to opl3, absolving
dmid(1) and other writers from constantly sending sync
events to force opl3 to synthesize audio in between events.
it reduces cpu usage and syscalls dramatically and enables
applications other than dmid(1) to modify its state in real
time, for instance to use it as a synth.

--- a/sys/src/games/opl3/opl3m.c

+++ b/sys/src/games/opl3/opl3m.c

@@ -13,7 +13,7 @@

 void

 usage(void)

 {

-	fprint(2, "usage: %s [-r rate] [file]\n", argv0);

+	fprint(2, "usage: %s [-s] [-r rate] [file]\n", argv0);

 	exits("usage");

 }

@@ -20,12 +20,15 @@

 void

 main(int argc, char **argv)

 {

-	int rate, n, r, v, fd, pfd[2];

-	uchar sb[65536 * 4], u[5];

-	double f, dt;

+	int rate, stream, n, r, v, fd, pfd[2];

+	uchar sb[64*1024], u[5];

+	double f;

+	vlong dt, T;

 	Biobuf *bi;

+	QLock slock;

 	fd = 0;

+	stream = 0;

 	rate = OPLrate;

 	ARGBEGIN{

 	case 'r':

@@ -33,6 +36,9 @@

 		if(rate <= 0 || rate > OPLrate)

 			usage();

 		break;

+	case 's':

+		stream = 1;

+		break;

 	default:

 		usage();

 	}ARGEND;

@@ -60,19 +66,44 @@

 	}

 	f = (double)OPLrate / rate;

 	dt = 0;

+	T = nsec();

+	if(stream){

+		switch(rfork(RFPROC|RFMEM)){

+		case -1:

+			sysfatal("rfork: %r");

+		case 0:

+			for(;;){

+				qlock(&slock);

+				n = OPLrate / 1e3;

+				T += n * (1e9 / OPLrate);

+				n *= 4;

+				opl3out(sb, n);

+				n = write(pfd[1], sb, n);

+				dt = (T - nsec()) / 1e6;

+				qunlock(&slock);

+				if(n <= 0)

+					break;

+				if(dt > 0)

+					sleep(dt);

+			}

+		}

+	}

 	while((n = Bread(bi, u, sizeof u)) > 0){

 		r = u[1] << 8 | u[0];

 		v = u[2];

 		opl3wr(r, v);

 		dt += (u[4] << 8 | u[3]) * f;

+		qlock(&slock);

 		while((n = dt) > 0){

 			if(n > sizeof sb / 4)

 				n = sizeof sb / 4;

 			dt -= n;

+			T += n * (1e9 / OPLrate);

 			n *= 4;

 			opl3out(sb, n);

 			write(pfd[1], sb, n);

 		}

+		qunlock(&slock);

 	}

 	if(n < 0)

 		sysfatal("read: %r");