Blackberry: Grab viewfinder frames from native window

Blackberry: Grab viewfinder frames from native window

Since the conversion from NV12 to RGB on the CPU does not scale
for larger photo/video resolutions, this patch uses a different
approach. It uses the low-level screen API to grab screenshots
of the native viewfinder window and provides them as QImage to
the QAbstractVideoSurface. Even for large resolutions this
is quite performant.

Change-Id: I59a7cbe6850b3b07575ea10026f3180cfd22e935
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>