[8u20] [SWT, Mac, Retina] FXCanvas draws badly when Scene is changed

[Stephen F Northover]

Hi Anton,

Please review the fix for FXCanvas drawing badly (shrinking) when a new 
scene is set on retina.

Jira: https://javafx-jira.kenai.com/browse/RT-35250
Webrev: See patch in the JIRA

Steve