GLPixelStorageModes: Safe and restore GL2ES3 modes, i.e. add missing GLES3 modes. GLStateTracker: Use proper GL names for enums
GLReadBufferUtil: Set PACK_ROW_LENGTH for ES3 as well - also set ReadBuffer to default.
Bug 975 - GLJPanel's OffscreenDrawable shall not double swap - Use default auto-swap mechanims

Refines commit 908ebd99d1eb57ce773a1fdd67c76886da86b9e6

Note that the test case decide whether to auto-swap (after read-pixels)
or not auto-swap (manual swap before read-pixels).

See UITestCase.swapBuffersBeforeRead(GLCapabilitiesImmutable chosenCaps):
   Determines whether the chosen GLCapabilitiesImmutable requires a swap-buffers before reading pixels.

   Usually one uses the default-read-buffer, i.e. GL.GL_FRONT for single-buffer
   and GL.GL_BACK for double-buffer GLDrawables
   and GL.GL_COLOR_ATTACHMENT0 for offscreen framebuffer objects.
   Here swap-buffers shall happen after calling reading pixels, the default.

   However, multisampling offscreen GLFBODrawables utilize swap-buffers to downsample
   the multisamples into the readable sampling sink.
   In this case, we require a swap-buffers before reading pixels.

   Returns:  chosenCaps.isFBO() && chosenCaps.getSampleBuffers()

+++

- GLJPanel:
  - Remove SurfaceUpdatedListener mechanism in favor of
    default auto-swap-buffer via GLDrawableHelper.
    This removes complexity.

  - postGL does not need to perform explicit swapBuffer operation,
    but rely on GLDrawableHelper and the default mechanism.
    This is also compatible w/ J2D backend.

  - Use GLDrawableHelper for setAutoSwapBufferMode(..) and getAutoSwapBufferMode()

+++

UnitTests:

- UITestCase:
  - Add 'boolean swapBuffersBeforeRead(GLCapabilitiesImmutable chosenCaps)'
    to determine whether swapBuffers() must occure before read-pixels. See above.

- GLReadBuffer00Base*
  - remove explicit addSnapshotGLEL/removeSnapshotGLEL

  - add TextRendererGLEL, to display frame-count and -dimension

  - SnapshotGLEL*
    - simply toggle auto-swap in their init(..) and dispose(..) method!

    - clear back-buffer if 'swapBuffersBeforeRead'
      to test whether the right buffer is being used for read-pixels.