Fix GLCanvas ctor shared GLContext: Use helper.setSharedContext(..) remove local shared ctx fields.

Regression of 7f7a23dd0ddf106e6f0c69fc2a05ff92ac56200e
Bug 776 GLContext Sharing: Add GLSharedContextSetter to GLJPanel
Bug 776 GLContext Sharing: Add note about driver stability (destruction order) ; Fix unit tests (Shared Gears, wait for created context and destruction order)

- Add note about driver stability (destruction order)
  - See GLSharedContextSetter: Don't destroy master context before slaves!

- Fix spec-overview.html#SHARED links, add link to GLSharedContextSetter in SHARED subsection.

- Fix unit tests (Shared Gears, wait for created context and destruction order)
  - The GearsObject sharing was completly bogus!
    It simply used the _same_ GLArrayDataServer instance for sharing,
    but it should use a _copy_ of the shared GLArrayDataServer while
    only preserving the VBO object!
    Fixed, while adding required methods to GLArrayDataServer.

  - Waiting for the created GLContext of a GLAutoDrawable required us
    to pass the latter _and_ check whether it's GLContext exists and is natively created.

  - Accomodated the destruction order - see above!