Changes:
- Add block size define.
- Set default to mean filtering.
- Add flag in AV1_COMP for bgsprite.
- Determine to use bgsprite or not based on firstpass metrics in
define_gf_group.
- Add outlier removal for bgsprite mean blending (off by default)

Results:
lowres: -0.009 avg_psnr, -0.029 ovr_psnr, -0.102 ssim

Clips with maximum gain:
- brdige_far_cif: -2.071 avg_psnr, -2.409 ovr_psnr, -1.977 ssim
- deadline_cif: -0.148 avg_psnr, -0.137 ovr_psnr, -0.113 ssim
- brdige_close_cif: +1.741 avg_psnr, -0.258 ovr_psnr, -2.534 ssim

Change-Id: I809406020f7786e49cc80329511e22d25379d7a2