RFR: JDK-8235954: [dmg] Default DMG background tiff of jpackage not retina ready

[Alexander Matveev]

Please review the jpackage fix for bug [1] at [2].

- Updated background image with one which supports retina display.

[1] https://bugs.openjdk.java.net/browse/JDK-8235954

[2] http://cr.openjdk.java.net/~almatvee/8235954/webrev.00/

Thanks,
Alexander