iOS: implement QIOSWindow::winId()

Change-Id: I3dd7accae43bcf7d4d6dfd8b272ab65d67bd935c
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
iOS: QIOSWindow::setParent()

Change-Id: I1a413d898d10b55a4d0653eae719f5bd909a01ec
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
iOS: Skip flushing child windows in QIOSBackingStore

We skip flushing raster-based child windows, to avoid the extra cost
of copying from the parent FBO into the child FBO. Since the child
is already drawn inside the parent FBO, it will become visible when
flushing the parent. The only case we end up not supporting is if the
child window overlaps a sibling window that's draws using a separate
QOpenGLContext.

Change-Id: Ib10414f4494747e5fe67f84b06575fe16ffddf96
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
iOS: Set touch point position in screen coords.

Previously the position was set in window coordinates,
which would break for non-fullscreen windows.

Change-Id: Iefa2f590c6d62b09fc3e7fe60a882c1acd33e029
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
iOS: Don't crash on landscape mode startup

fromPortraitToPrimary is called from the QIOSScreen
constructor. This is probably to early to call QGuiApplication
functions.

Change-Id: I882304fd641df13dc530491990245ba9ad495377
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>