QNX: Support window translucency

A few tactical changes were necessary to do this:

* The root window's buffer must be the full size of the window in
  order for transparency to work. We now transition this buffer
  to full-size upon first request of some QWindow to be translucent.

  At the same time the root window is transitioned to being full-
  screen, we also set its alpha blending mode to Source-Over in
  order to permit lower z-ordered apps to show through.

* Set the root window's buffer position to (0, 0) to avoid a momentary
  flicker of unpainted pixels on the far right-hand border as a
  translucent window comes on-screen.

* Use Source-Over alpha blending for normal child windows too.

The QtGui core logic for setting the backing surface pixel format to
something with an alpha channel if Qt::WA_TranslucentBackground is
set on the toplevel QWidget already accomplished the remainder of
the work.

Change-Id: I1c2b31aa6323b4555fa194313f0f5e06ada494fc
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
QNX: support underlain foreign windows

Previously, any foreign window was treated as an overlay, with its
z-order being forcibly set above the native Qt windows.

This change adjusts the strategy to treat foreign windows whose initial
z-order is less than -1 (the default z-order used by mmrender for its
overlay surfaces) as underlays; otherwise the foreign window is treated
as an overlay.

Change-Id: Ifbcfd3a956965fa1a347076e9845afa50a81edfd
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Fix crash in QCococaWindow::childWindowAt().

Task-number: QTBUG-31297

Change-Id: I96f4652e410334fae54a0012ed917a965becfe5c
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@digia.com>