[EGL/X11]Context: Enhance DEBUG output for failed makeCurrentImpl(..)
ProxySurface: Add 'NativeSurface getUpstreamSurface()' ; ProxySurfaceImpl: Don't cache 'displayHandle'

- ProxySurface: Add 'NativeSurface getUpstreamSurface()' allowing querying direct access of a backing surface representing this instance.
  - Use case: EGLWrappedSurface
  - Default impl. returns null

- ProxySurfaceImpl: Don't cache 'displayHandle'
  - getDisplayHandle() is 'final', no more 'shortcut' code allowed
    due to re-association incl. display handle.

  - See commit b738983638703bb721ee4c9820c8ef43e2252e73
WGLContext: Enhance DEBUG output for failed makeCurrentImpl(..)