Enable segmentation on bgsprite.
Enable segmentation on bgsprite.

Changes:
- Enable segmentation by default on bgsprite.
- Turn off global motion estimation by default on bgsprite.
- Segment in 4x4 blocks and use temporal filter on foreground blocks.

Change-Id: I15ccfd908d5772287858ceb7b1aca44e6a9b1071

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom