font/sfnt: rename a "stack" field to "argStack".

This will lessen the diff in a subsequent commit, which introduces a
call stack.

Change-Id: I478857675f47b53e6bd3ee270087a00b9d1d4d34
Reviewed-on: https://go-review.googlesource.com/38279
Reviewed-by: Nigel Tao <nigeltao@golang.org>
1 file changed