8u40 review request: RT-34467 huge memory consumption resizing transparent window

[Jim Graham]

webrev: http://cr.openjdk.java.net/~flar/RT-34467/webrev.00/
Jira: https://javafx-jira.kenai.com/browse/RT-34467

This builds on the recent fixes for RT-38923 by adding a mechanism to reuse old buffers when they are large enough.

Both test cases in RT-34467 now seem to be fairly well behaved (with both default and 1000m heap parameters)...

		...jim