Fix UIDemo runtime exceptions using com.sun.java.swing.plaf.gtk.GTKLookAndFeel
Ubuntu OpenJDK UIManager.getSystemLookAndFeelClassName() return this LAF name.

Signed-off-by: Xerxes Rånby <xerxes@zafena.se>
fixed a bug that wasn't disabling stipple
Merge pull request #12 from xranby/GTKLookAndFeel

Fix UIDemo runtime exceptions using GTKLookAndFeel
moved the Swing hierarchy verification into the G2DGLCanvas
as far as I can tell, this odd behavior of gl.glIsBuffer() is not an issue anymore
added a more methodical way to notify the user of unsupported features, log it only the first time
clarified that helper implementations should clear their stacks in setG2D()