libvpx

Expose params min-gf-interval/max-gf-interval

Adds two new vp9 parameters --min-gf-interval and --max-gf-interval
to enable testing based on frequency of alt-ref frames.

Also adds a unit-test to test enforcement of min-gf-interval.

For both these parameters the default value is 0, which indicates
they are picked by the encoder, based on resolution and framerate
considerations. If they are greater than zero, the specified
parameter is honored.

(Additional note by paulwilkins)
Note that there is a slight oddity in that key frames are also GFs and
considered part of  GF only group. However they are treated as not
being part of an arf group because for arf groups the previous GF is
assumed to be the terminal or overlay frame for the previous group.

(end note)

Change-Id: Ibf0c30b72074b3f71918ab278ccccc02a95a70a0