font/sfnt: fix argStack size check for moveto ops.

The *moveto ops don't take multiple args or arg pairs. For example, in
http://wwwimages.adobe.com/content/dam/Adobe/en/devnet/font/pdfs/5177.Type2.pdf
page 16, the rmoveto args are listed as:
dx1 dy1
which is not the same as rlineto's args, which can be repeated:
{dxa dya}+

Change-Id: I5d13f686e604955909eb0b7e52f20ce5f0522c5d
Reviewed-on: https://go-review.googlesource.com/38289
Reviewed-by: David Crawshaw <crawshaw@golang.org>
1 file changed