shiny/widget: have Text.Paint only draw visible lines of text.
shiny/widget: have Text.Paint only draw visible lines of text.

Change-Id: If7b59eccd96516953dc5c471683bf30e06220a8b
Reviewed-on: https://go-review.googlesource.com/24441
Reviewed-by: David Crawshaw <a2f0bf29257797dd66800b4fbb9945c3e216c004@golang.org>

golang/exp,golang/exp,golang/exp