need a way to get the size of an image more elegantly than catting it
and extracting the image size in an ad-hoc manner.  the code isn't
very efficient since the whole image is read.

Reference: /n/atom/patch/applied/imagesize
Date: Wed Feb 19 03:58:20 CET 2014
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/imgsize.c	Thu Jan  1 00:00:00 1970
+++ /sys/src/cmd/imgsize.c	Wed Feb 19 03:57:34 2014
@@ -0,0 +1,61 @@
+#include <u.h>
+#include <libc.h>
+#include <draw.h>
+#include <memdraw.h>
+
+int
+Rfmt(Fmt *f)
+{
+	Rectangle r;
+
+	r = va_arg(f->args, Rectangle);
+	return fmtprint(f, "%d %d %d %d", r.min.x, r.min.y, r.max.x, r.max.y);
+}
+
+static Rectangle
+findrect(int fd)
+{
+	Memimage *m;
+	Rectangle r;
+
+	m = readmemimage(fd);
+	if(m == nil)
+		sysfatal("imgsize: readmemimage: %r");
+	r = m->r;
+	freememimage(m);
+	return r;
+}
+
+void
+usage(void)
+{
+	fprint(2, "usage: imgsize\n");
+	exits("usage");
+}
+
+void
+main(int argc, char **argv)
+{
+	int i, fd;
+
+	ARGBEGIN{
+	default:
+		usage();
+	}ARGEND
+
+	fmtinstall('R', Rfmt);
+	if(argc > 0){
+		for(i = 0; i < argc; i++){
+			fd = open(argv[i], OREAD);
+			if(fd == -1)
+				sysfatal("imgsize: open: %r");
+			if(argc>1)
+				print("%s	%R\n", argv[i], findrect(fd));
+			else
+				print("%R\n", findrect(fd));
+			close(fd);
+		}
+	}else
+		print("%R\n", findrect(0));
+	exits("");
+}
--- /sys/man/1/imgsize	Thu Jan  1 00:00:00 1970
+++ /sys/man/1/imgsize	Wed Feb 19 03:57:34 2014
@@ -0,0 +1,21 @@
+.TH IMGSIZE 1
+.SH NAME
+imagesize \- print image rectangle
+.SH SYNOPSIS
+.B imgsize
+[
+.I file ...
+]
+.SH DESCRIPTION
+Print the enclosing rectangle of each input image file
+without punctuation.
+If no files are given on the command line, standard
+input is read.
+If multiple files are given, each rectangle is preceeded
+by the corresponding file name.
+.SH SOURCE
+.B /sys/src/cmd/imgsize.c
+.SH "SEE ALSO"
+.IR draw (2),
+.IR image (6),
+.IR memdraw (2).