[OpenJDK 2D-Dev] RFR: 7145508: java.awt.GraphicsDevice.get/setDisplayMode behavior is incorrect when no display is present

[Phil Race]

http://cr.openjdk.java.net/~prr/7145508/
https://bugs.openjdk.java.net/browse/JDK-7145508

If you physically disconnect the monitor and run headful, then on X, the 
XR&R
extension will return NULL from querying the display config.
In such a case we can't say we are headless since X is present and 
operational,
and we are way past the point of deciding we are headless. Plus this 
disconnection
could happen at any time. In this case just return a default mode based 
on the
X window root properties.

A regression test is not provided because this is covered by JCK tests
and is configuration specific.

-phil.