FYI: Minor update to shapshot methods

[Kevin Rushforth]

http://javafx-jira.kenai.com/browse/RT-21570

The newly added snapshot (render to image) API takes an optional Image 
argument, which if non-null, should be updated in place with the result 
of the snapshot. This was not implemented in the first version of 
snapshot since it relies upon Image Ops which was just completed. Now 
that Image Ops is in, the snapshot API will need to be slightly 
modified. The parameter and return value of the 4 snapshot methods will 
change to taking a WritableImage subclass of Image, both for type 
safety, and in order to implement updating an existing image in place.

-- Kevin