Supports only raw NV12 input.

Example use:
./vaapi_encode 1920 1080 test.yuv test.h264
Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Signed-off-by: Liu, Kaixuan <kaixuan.liu@intel.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>