QNX: avoid composition when possible

Window buffers are currently always created with an alpha channel when
the underlying display supports them. This is the correct thing to to
in order to support translucency. However, the cost is that the
composition manager unconditionally performs blending even when the
use-case in the GUI doesn't make use of it. That is rather expensive,
particularly on hardware which doesn't have extremely efficient
dedicated 2D blitting hardware.

This patch adds inspection of the user's requested window format
and--if no blending is desired--sets an additional Screen window
property which permits Screen to skip blending on the window surface.

Change-Id: I83c100a351a00ef0e8bea1b1d1fec10b7218d1de
Reviewed-by: Kevin Krammer <kevin.krammer@kdab.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>
Reviewed-by: Frank Osterfeld <frank.osterfeld@kdab.com>
Reviewed-by: Rafael Roquetto <rafael.roquetto@kdab.com>
Doc: solved link problem for dombookmarks.qdoc

Task-number: QTBUG-34749
Change-Id: I2f2fbf16314e069ab04e5c5ab4bcef9780c008a2
Reviewed-by: Venugopal Shivashankar <venugopal.shivashankar@digia.com>
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>