UpstreamSurfaceHook: Add 'NativeSurface getUpstreamSurface()' (from EGLUpstreamSurfaceHook) to generalize ProxySurfaceImpl.getUpstreamSurface()
Minor: API-Doc / DEBUG Typo, GDISurface: Make fields private
Bug 741 HiDPI: Add ScalableSurface interface to get/set pixelScale w/ full OSX impl.

Add ScalableSurface interface
  - To set pixelScale before and after realization
  - To get pixelScale

  - Implemented on:
    - NEWT Window
      - Generic impl. in WindowImpl
      - OSX WindowDriver impl.
        - Also propagetes pixelScale to parent JAWTWindow if offscreen (NewtCanvasAWT)
      - AWT WindowDriver impl.

    - JAWTWindow / OSXCalayer
      - AWT GLCanvas
      - AWT GLJPanel
      - NEWTCanvasAWT:
        - Propagates NEWT Window's pixelScale to underlying JAWTWindow

    - WrappedSurface for pixelScale propagation
      using offscreen drawables, i.e. GLJPanel

    - Generic helper in SurfaceScaleUtils (nativewindow package)

  - Fully implemented on OSX

  - Capable to switch pixelScale before realization,
    i.e. native-creation, as well as on-the-fly.

  - Impl. uses int[2] for pixelScale to support
    non-uniform scale.

Test cases:
  - com.jogamp.opengl.test.junit.jogl.demos.es2.newt.TestGearsES2NEWT
  - com.jogamp.opengl.test.junit.jogl.demos.es2.awt.TestGearsES2AWT
  - com.jogamp.opengl.test.junit.jogl.demos.es2.awt.TestGearsES2GLJPanelAWT
  - com.jogamp.opengl.test.junit.jogl.demos.es2.newt.TestGearsES2NewtCanvasAWT

  - Press 'x' to toggle HiDPI
  - Commandline '-pixelScale <value>'
  - Added basic auto unit test (setting pre-realization)