8u20 review request: RT-33294 - Canvas PixelWriter slow to fill canvas pixel by pixel

[Jim Graham]

Jira: https://javafx-jira.kenai.com/browse/RT-33294
webrev: http://cr.openjdk.java.net/~flar/RT-33294/webrev.00/
Tested using the submitted test case and then also using all of the 
Canvas toys in rt-closed...

                        ...jim