[9] Review request for 8137050: Support Linux settings for HiDPI scaling

[Jim Graham]

bug: https://bugs.openjdk.java.net/browse/JDK-8137050
webrev: http://cr.openjdk.java.net/~flar/JDK-8137050/webrev-00/

The order of taking scaling parameters in order of higher to lower priority is:
- -Dglass.gtk.uiScale system property
- Same property in the environment
- GDK_SCALE in the environment (optional in Gnome)
- g_settings (the typical way to set scaling...)

As noted in the bug report, I'll delete the old lines in the Screen constructor in glass_screen.cpp that are currently 
commented out if there are no other changes noted in the review...

I was able to run dev-build-full.sh and got the same errors as I did without the fix.

I ran SwingNode toys and none of them scaled despite the fact that I was running on jdk9 and AWT was supposed to support 
scaling, but they worked fine in that I could manipulate all of the Swing controls.

			...jim