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lib(mem)draw: assert chan type uniqueness except for CIgnore

without this check it's possible to create images with multiple
channels of the same type, contrary to the documented image(6)
format.

--- a/sys/src/libdraw/chan.c

+++ b/sys/src/libdraw/chan.c

@@ -41,11 +41,11 @@

 {

 	char *p, *q;

 	ulong c;

-	int t, n, d;

+	int usedt, t, n, d;

 	c = 0;

-	d = 0;

-	p=s;

+	d = usedt = 0;

+	p = s;

 	while(*p && isspace(*p))

 		p++;

@@ -53,6 +53,8 @@

 		if((q = strchr(channames, p[0])) == nil) 

 			return 0;

 		t = q-channames;

+		if(t != CIgnore && usedt & (1<<t))

+			return 0;

 		if(p[1] < '0' || p[1] > '9')

 			return 0;

 		n = p[1]-'0';

@@ -59,6 +61,7 @@

 		d += n;

 		c = (c<<8) | __DC(t, n);

 		p += 2;

+		usedt |= 1<<t;

 	}

 	if(d==0 || (d>8 && d%8) || (d<8 && 8%d))

 		return 0;

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

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

@@ -142,7 +142,7 @@

 	int d;

 	int t, j, k;

 	ulong cc;

-	int bytes;

+	int bytes, usedt;

 	if((d = chantodepth(chan)) == 0) {

 		werrstr("bad channel descriptor");

@@ -153,9 +153,10 @@

 	i->chan = chan;

 	i->flags &= ~(Fgrey|Falpha|Fcmap|Fbytes);

 	bytes = 1;

+	usedt = 0;

 	for(cc=chan, j=0, k=0; cc; j+=NBITS(cc), cc>>=8, k++){

 		t=TYPE(cc);

-		if(t < 0 || t >= NChan){

+		if(t < 0 || t >= NChan || (t != CIgnore && usedt & (1<<t))){

 			werrstr("bad channel string");

 			return -1;

 		}

@@ -171,6 +172,7 @@

 		i->shift[t] = j;

 		i->mask[t] = (1<<NBITS(cc))-1;

 		i->nbits[t] = NBITS(cc);

+		usedt |= 1<<t;

 		if(NBITS(cc) != 8)

 			bytes = 0;

 	}

-- 

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