Before this commit set_frame_size decided on the frame size to use and
then set it. It has been changed to just take an arbitrary size to make
it more flexible.

The decision of what size to use has been moved to the new
set_scaled_size and existing av1_calculate_superres_size.

setup_frame_size implements the old logic of getting the scaled size,
then superres size, then setting the frame to use it.

This change will make post-encode upscaling possible using
set_frame_size since it can now be given a target resolution.

Change-Id: I2f0f20740a54f751bc9963f279d6bcfdfb2722c2