FX 8u20 review request: RT-25249 ImageInput does not update for changes to WritableImage

[Jim Graham]

Jira: https://javafx-jira.kenai.com/browse/RT-25249
webrev: http://cr.openjdk.java.net/~flar/RT-25249/webrev.00/

The code was taken as a boilerplate from the ImageView code...

                        ...jim