For DeinterlacingBob mode with rate=field, the frame number of output
should equal 2x input total since only intra deinterlace is used.

Currently for "backward_ref = 0, rate = field", extra_delay is
introduced. Due to the async without flush, frame number of output is
[expected_number - 2].

Specifically, if the input only has 1 frame, the output will be empty.

Add deint_vaapi_request_frame for deinterlace_vaapi, send NULL frame
to flush the queued frame.

For 1 frame input in Bob mode with rate=field,
before patch: 0 frame;
after  patch: 2 frames;

ffmpeg -hwaccel vaapi -hwaccel_device /dev/dri/renderD128
        -hwaccel_output_format vaapi -i input.h264 -an -vf
        deinterlace_vaapi=mode=bob:rate=field -f null -
Tested-by: Mark Thompson <sw@jkqxz.net>
Reviewed-by: Mark Thompson <sw@jkqxz.net>
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Haihao Xiang <haihao.xiang@intel.com>