Layer based rate control for CBR mode.
This patch adds a buffer-based rate control for temporal layers, under CBR mode. Added vpx_temporal_scalable_patters.c encoder for testing temporal layers, for both vp9 and vp8 (replaces the old vp8_scalable_patterns). Updated datarate unittest with tests for temporal layer rate-targeting. Change-Id: I8900a854288b9354d9c697cfeb0243a9fd6790b1