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libmemdraw: never point Buffer.alpha to nil (thanks mischief)

the boolcopy optimization doesnt doesnt use Buffer.alpha, tho
the debug function dumpbuf() still can dereference it. to keep
it simple, always have Buffer.alpha point to the channel or
&ones when not used.

--- a/sys/src/libmemdraw/draw.c

+++ b/sys/src/libmemdraw/draw.c

@@ -405,7 +405,7 @@

 	uchar	*red;

 	uchar	*grn;

 	uchar	*blu;

-	uchar	*alpha;

+	uchar	*alpha;	/* is &ones when unused, never nil */

 	uchar	*grey;

 	ulong	*rgba;

 	int	delta;	/* number of bytes to add to pointer to get next pixel to the right */

@@ -1677,7 +1677,7 @@

 		b.grey = buf+copyalpha;

 		b.red = b.grn = b.blu = buf+copyalpha;

 		b.delta = copyalpha+1;

-DBG print("alpha %x grey %x\n", b.alpha ? *b.alpha : 0xFF, *b.grey);

+DBG print("alpha %x grey %x\n", *b.alpha, *b.grey);

 	}else{

 		b.blu = buf+copyalpha;

 		b.grn = buf+copyalpha+1;

@@ -1695,7 +1695,7 @@

 {

 	Memimage *img;

 	int i, isalpha, isgrey, nb, delta, dx, adelta;

-	uchar ff, *red, *grn, *blu, *grey, *alpha;

+	uchar *red, *grn, *blu, *grey, *alpha;

 	ulong u, mask;

 	img = p->img;

@@ -1714,11 +1714,8 @@

 	isgrey = img->flags&Fgrey;

 	adelta = src.delta;

-	if(isalpha && (alpha == nil || alpha == &ones)){

-		ff = 0xFF;

-		alpha = &ff;

+	if(isalpha && alpha == &ones)

 		adelta = 0;

-	}

 	if((img->flags&Fbytes) != 0){

 		int ogry, ored, ogrn, oblu, oalp;

@@ -1829,7 +1826,8 @@

 	USED(s);

 	q = p->bytermin + y*p->bwidth;

 	b.red = q;	/* ptr to data */

-	b.grn = b.blu = b.grey = b.alpha = nil;

+	b.grn = b.blu = b.grey = nil;

+	b.alpha = &ones;

 	b.rgba = (ulong*)q;

 	b.delta = p->img->depth/8;

 	return b;

@@ -1947,7 +1945,8 @@

 	}

 	b.red = buf;

-	b.blu = b.grn = b.grey = b.alpha = nil;

+	b.blu = b.grn = b.grey = nil;

+	b.alpha = &ones;

 	b.rgba = (ulong*)buf;

 	b.delta = 0;