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cfilter: properly handle alpha images

	images were transformed to XRGB32 whatsoever instead of
	checking if the input had an alpha channel. This lead to
	wrong colors in the output image.
	We now check the input image chan and transform to XRGB32 or
	ARGB32 based on that.

--- a/cfilter.c

+++ b/cfilter.c

@@ -72,11 +72,22 @@

 	{ "tint", tint },

 };

+int

+isalpha(ulong chan)

+{

+	ulong t;

+

+	for(t = chan; t; t >>= 8)

+		if(TYPE(t) == CAlpha)

+			return 1;

+	return 0;

+}

+

 void

 main(int argc, char *argv[])

 {

 	Memimage *o, *i;

-	int w, h, p, n;

+	int w, h, p, n, a;

 	uchar *buf;

 	Filter *f;

 	float factor;

@@ -110,7 +121,8 @@

 	o = readmemimage(0);

 	if(o==nil)

 		sysfatal("readmemimage: %r");

-	i = allocmemimage(o->r, XRGB32);

+	a = isalpha(o->chan);

+	i = allocmemimage(o->r, a ? ARGB32 : XRGB32);

 	memimagedraw(i, i->r, o, o->r.min, nil, ZP, S);

 	freememimage(o);

 	w = Dx(i->r);

@@ -124,7 +136,7 @@

 	freememimage(i);

 	for(p = 0; p < n; p+=4)

 		f->filter(buf+p, factor);

-	print("   x8r8g8b8 %11d %11d %11d %11d ", 0, 0, w, h);

+	print("   %c8r8g8b8 %11d %11d %11d %11d ", a ? 'a' : 'x', 0, 0, w, h);

 	write(1, buf, n);

 	exits(nil);

 }