shiny/text: treat maxWidth <= 0 as infinite, as per the docs.
shiny/text: treat maxWidth <= 0 as infinite, as per the docs.

Change-Id: I02d8d68c672f0152460a1e069eeb59eacf2c284f
Reviewed-on: https://go-review.googlesource.com/19114
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/exp,golang/exp,AkihiroSuda/golang-exp-aspectgo,AkihiroSuda/golang-exp-aspectgo,golang/exp,AkihiroSuda/golang-exp-aspectgo