shiny/imageutil: new package.

Add an imageutil.Border function, for drawing outlined-but-not-filled
rectangles, such as widget borders, checkboxes, table cells, etc.

Change-Id: I1e55e5fe492a5d2bb4df6f03ab8d14c9cb9db67d
Reviewed-on: https://go-review.googlesource.com/24632
Reviewed-by: David Crawshaw <crawshaw@golang.org>
3 files changed