The vbp thresholds are set seperately for boosted/non-boosted
superblocks according to their segment_id. This way we don't
have to force the boosted blocks to split to 32x32.

Speed 6 RTC set borg test result showed some quality gains.
Overall PSNR: +0.199%; Avg PSNR: +0.245%; SSIM: +0.802%.
No speed change was observed.

Change-Id: I37c6643a3e2da59c4b7dc10ebe05abc8abf4026a