shiny/widget: add a RunWindow function.

This connects a screen.Window with a widget tree.

Also add an Image widget and an imageview example program.

Also make UniformClass.Paint not panic on a nil color.Color.

Change-Id: Ib3e4eb076e5e15909243b6cc44d22c73f7bfe58c
Reviewed-on: https://go-review.googlesource.com/22107
Reviewed-by: David Crawshaw <crawshaw@golang.org>
4 files changed