Add control API VP9E_SET_TARGET_LEVEL that allows the encoder to
control the output bitstream level and/or keep level related
statistics.
Usage:
255 do not care about level (default)
0 keep level related stats only
10 target for level 1
11 target for level 1.1
.
.
.
62 target for level 6.2
Usage for vpxenc:
--target-level=0/255/10/11...
Change-Id: I31d1aeca19358b893e7577b4e63748c8e614034a