if the channdesc is not CY, then the calculated size in png
is wrong by a factor of the number of bytes per pixel.  a more
invasive but perhaps better patch would create a function
chantobpp and use that where required.

Reference: /n/sources/patch/applied/pngsize
Date: Sun Aug  9 02:32:11 CES 2009
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/jpg/png.c	Sun Aug  9 02:29:50 2009
+++ /sys/src/cmd/jpg/png.c	Sun Aug  9 02:29:49 2009
@@ -214,6 +214,20 @@
 	if(nineflag){
 		chantostr(buf, outchan);
 		len = (c->r.max.x - c->r.min.x) * (c->r.max.y - c->r.min.y);
+		switch(c->chandesc){
+		case CY:
+			len *= 1;
+			break;
+		case CYA16:
+			len *= 2;
+			break;
+		case CRGB24:
+			len *= 3;
+			break;
+		case CRGBA32:
+			len *= 4;
+			break;
+		}
 		if(c->chanlen != len)
 			fprint(2, "%s: writing %d bytes for len %ld chan %s\n",
 				argv0, c->chanlen, len, buf);