8u20 review request: RT-37449 - use of Canvas PixelWriter leaves graphics in SRC mode

[Jim Graham]

webrev: http://cr.openjdk.java.net/~flar/RT-37449/webrev.00/
Jira: https://javafx-jira.kenai.com/browse/RT-37449

Simple fix, details in Jira comments...

                ...jim