This commit moves the av1_scale_if_required functions from
av1/encoder/encoder to av1/common/resize. They take the
scale_and_extend_frame_nonnormative function with them.

This will be useful for resizing received frames in the decoder for the
FRAME_SUPERRES experiment.

Change-Id: I83afc3d97ff8ebce5ecb2e1205ba6c271156cb31