vp9: Updates to SVC sample encoder.

Allow for passing in the layer bitrates at command line.
Fix to allow passing in bitrate for each spatial-temporal layer.

Change to some default values for 1 pass cbr mode:
spatial scale and qp-max/min.

Small fixes to some build warnings.

Change-Id: I3f9a776262712480a6570bb863a835b2fc49935a