Correct the usage of native size
There seems to be some confusion about the use of nativeSize in the backend. This MR attempt to fix that using a couple of commits.
The first commit makes the m_nativeSize variable in the backend not affected by orientation settings but by the texture orientation only. This fixes the calculation of VideoOutput's contentRect and the normalized point calculation .
The second commit corrects the calculation of adjustedViewport(). It has incorrectly returned the scaled size of the native size by the bounding rect. It now just returns the native size directly as that's what's expected by VideoOutput.
 But, as the backend has the concept of texture orientation, apps might need to do some further work to make sure the normalized points are in the correct orientation.