examples/hw_decode: Add a HWAccel decoding example.

Works with VAAPI, VDPAU, DXVA2 and D3D11VA.

Signed-off-by: Liu, Kaixuan <kaixuan.liu@intel.com>
Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Reviewed-by: Steven Liu <lingjiujianke@gmail.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>