CQ Mode

The merge includes hooks to for CQ mode and other code
changes merged from the test branch.

CQ mode attempts to maintain a more stable quantizer within a clip
whilst also trying to adhere to a guidline maximum bitrate.

The existing target data rate parameter is used to specify the
guideline maximum bitrate.

A new parameter allows the user to specify a target CQ level.

For normal (non kf/gf/arf) frames, the quantizer will not drop BELOW the
user specified value (0-63). However, in some cases the encoder may
choose to impose a target CQ that is above that specified by the user,
if it estimates that consistent use of the target value is not compatible
with guideline maximum bitrate.

Change-Id: I2221f9eecae8cc3c431d36caf83503941b25e4c1