8u20 review request: RT-36016 dirty region problems with large blurs

[Jim Graham]

webrev: http://cr.openjdk.java.net/~flar/RT-36016/webrev.00/
Jira: https://javafx-jira.kenai.com/browse/RT-36016

This is a regression caused by the recent work on blurs/shadows on retina...

                        ...jim