Fixed point reference picture scaling

Fixed point reference picture scaling

Fixed point scaling factors are calculated once for each
reference frame by using integer division. Otherwise fixed point
scaling routines are used in all scaling calculations. This makes it
possible to calculate fixed point scaling factors on device driver
software and pass them to hardware and thus avoid division on hardware.

TODO:
 - Missing check for maximum frame dimensions
   (currently scaling uses 14 bits)
 - Missing check for maximum scaling ratio
   (upscaling 16:1, downscaling 2:1)

Problems:
 - Straightforward fixed point implementation can cause error +-1
   compared to integer division (i.e. in x_step_q4). Should only
   be an issue for frames larger than 16k.

Change-Id: I3cf4dabd610a4dc18da3bdb31ae244ebaf5d579c