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games/gb: fix reversed audio channels
The high bits correspond to the left channel, and the low bits to the
right channel.

Reference: https://gbdev.io/pandocs/#sound-control-registers

Tested with pokemon crystal.

--- a/sys/src/games/gb/apu.c

+++ b/sys/src/games/gb/apu.c

@@ -115,13 +115,13 @@

 		if(i == 2 ? ((reg[NR30] & 0x80) == 0) : ((*sndch[i].env & 0xf8) == 0))

 			continue;

 		v = sndch[i].samp * 2 - 15;

-		if((cnth & 1<<i) != 0)

-			ov0 += v;

 		if((cnth & 1<<4<<i) != 0)

+			ov0 += v;

+		if((cnth & 1<<i) != 0)

 			ov1 += v;

 	}

-	ov0 *= 1 + (cntl & 7);

-	ov1 *= 1 + (cntl >> 4 & 7);

+	ov0 *= 1 + (cntl >> 4 & 7);

+	ov1 *= 1 + (cntl & 7);

 }

 static void