The bounding box of a string does not necessarily contain the origin.
Prior to this commit, BoundString(etc, "x") would call grow exactly
once, with the first argument being the (empty) zero rectangle.
Change-Id: Id7d4f6c250ac0749f6dae19d11f4e97f9c6f45bc
Reviewed-on: https://go-review.googlesource.com/34674
Reviewed-by: Dave Day <djd@golang.org>
