examples: Add a VA-API transcode example.

Usage is:
./vaapi_transcode input_stream codec output_stream
For example:
- ./vaapi_transcode input.mp4 h264_vaapi output_h264.mp4
- ./vaapi_transcode input.mp4 vp8_vaapi output_vp8.ivf

Does not handle resolution changes on the input stream.

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>