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pcmconv: revert previous change, fix dither clipping

previous change had forgot how dithering works... m(

we *really* add a random signal when *reducing* the number
of bits. just make sure we do not overflow like in
mixin().

--- a/sys/src/cmd/audio/pcmconv/pcmconv.c

+++ b/sys/src/cmd/audio/pcmconv/pcmconv.c

@@ -209,12 +209,13 @@

 {

 	static ulong prnd;

-	if(ibits >= 32 || ibits >= obits)

+	if(ibits >= 32 || obits >= ibits)

 		return;

 	while(count--){

 		prnd = (prnd*0x19660dL + 0x3c6ef35fL) & 0xffffffffL;

-		*y++ += ((int)prnd) >> ibits;

+		*y = clip((vlong)*y + ((int)prnd >> ibits));

+		y++;

 	}

 }