Review request for 7124236: [macosx] Some components lost shadows after the latest fix of translucent windows.

[Dmitry Cherepanov]

The cause of the problem with broken shadows is that currently 
NSWindow's background color is [NSWindow clearColor] and it makes 
shadows invisible. The fix implements Mike's suggestion and now we draw 
pure transparent pixels into a CALayer so that the native background 
color will be visible through transparent areas of the CALayer.

http://cr.openjdk.java.net/~dcherepanov/7124236/webrev.0/

Thanks,
Dmitry