Note: This feature is still in development.

Add an option for the encoder to decide the resolution
at which to encode each frame.

Each KF/GF/ARF goup is tested to see if it would be
better encoded at a lower resolution. At present, each
KF/GF/ARF is coded first at full-size and if the coded
size exceeds a threshold (twice target data rate) at
the maximum active Q then the entire group is encoded
at lower resolution.

This feature is enabled in vpxenc by setting:
  --resize-allowed=1

In addition, if the vpxenc command line also specifies
valid frame dimensions using:
  --resize-width=XXXX & --resize_height=YYYY
then *all* frames will be encoded at this resolution.

Change-Id: I13f341e0a82512f9e84e144e0f3b5aed8a65402b