We reduce max stack size from 16 to 8.

Memory reduction:
- peak usage for 1080p video: 2.328 GB → 1.788 GB
- sizeof ref_mv_stack: 6144 → 3072
- sizeof(MB_MODE_INFO_EXT): 6456 → 3384
- sizeof(PICK_MODE_CONTEXT):8056 → 5000
- sizeof(PC_TREE): 201440 → 125040

Compression performance is roughly neutral:
- AWCY objective-1-fast: +0.03
- Google lowres: 0.0
- Google midres: -0.006

BUG=aomedia:940

Change-Id: Ifd38359c58e40b1c94552c5034618da8ce510f62