qsv: adding Multi Frame Encode support

qsv: adding Multi Frame Encode support

Starting from API 1.25 helps to improve performance of the simultaneous
encode, 1:N scenario, like:

./avconv  -y -hwaccel qsv -c:v h264_qsv -r 30000/1001 -i
~/bbb_sunflower_1080p_60fps_normal.mp4  -vframes 600 -an \
    -filter_complex "split=2[s1][s2]; [s1]scale_qsv=1280:720[o1];
[s2]scale_qsv=960:540[o2]" \
    -map [o1] -c:v h264_qsv -b:v 3200k -minrate 3200k -maxrate 3200k -f
rawvideo /tmp/3200a.264 \
    -map [o2] -c:v h264_qsv -b:v 1750k -minrate 1750k -maxrate 1750k -f
rawvideo /tmp/1750a.264

Signed-off-by: Maxym Dmytrychenko <maxim.d33@gmail.com>