Refine VP8 encode middleware

BZ: 103440

Add VP8 encoder option in mix_encoder test tool;
Add render slice parameter for vp8 encode;
Change ref frame management for vp8 encode;
Clean up unused code for vp8 encode.

Change-Id: If57158788baa18a71fdd57ccbcef93e5ac552f39
Signed-off-by: Liu, Bolun <bolun.liu@intel.com>
Reviewed-on: http://android.intel.com:8080/109160
Reviewed-by: cactus <cactus@intel.com>
Reviewed-by: Ding, Haitao <haitao.ding@intel.com>
Reviewed-by: Shi, PingX <pingx.shi@intel.com>
Tested-by: Shi, PingX <pingx.shi@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>
3 files changed