-Make Rate control work for SVC 1 pass CBR mode.
-Added temporal layering mode.
-Fixed bug in non-rd variance partition.
-Modified/updated the sample encoders (vp9_spatial_svc_encoder, vpx_temporal_svc_encoder).
-Added datarate unittest(s) for 1 pass CBR SVC.

Change-Id: Ie94b1b68a56ea1267b5087c625e5df04def2ee48