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: I9cb6cce2494390ae6096ee17774af7fb9308bde7