qsv transcoding doesn't work with a large look_ahead_depth

[Zhong,Li]

Summary of the bug:
qsv transcoding doesn't work with a large look_ahead_depth (look_ahead_depth > 47 doses't work in the example as below).
Report an error "Cannot allocate memory".

How to reproduce:

./ffmpeg -hwaccel qsv -c:v h264_qsv -i ./fate-suite/h264-conformance/BA_MW_D.264 -c:v h264_qsv -look_ahead 1 -look_ahead_depth 47  test.h264

ffmpeg started on 2018-06-15 at 17:00:55
Report written to "ffmpeg-20180615-170055.log"
ffmpeg version N-91267-g238a8ae Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.9) 20160609
  configuration: --enable-libmfx --enable-libx264 --enable-gpl --disable-optimizations --enable-debug=3
  libavutil      56. 18.102 / 56. 18.102
  libavcodec     58. 20.101 / 58. 20.101
  libavformat    58. 17.100 / 58. 17.100
  libavdevice    58.  4.100 / 58.  4.100
  libavfilter     7. 25.100 /  7. 25.100
  libswscale      5.  2.100 /  5.  2.100
  libswresample   3.  2.100 /  3.  2.100
  libpostproc    55.  2.100 / 55.  2.100
Input #0, h264, from '/root/ffmpeg_fate_suite_lastest/fate-suite/h264-conformance/BA_MW_D.264':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (Constrained Baseline), yuv420p(progressive), 176x144, 25 fps, 25 tbr, 1200k tbn, 50 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (h264_qsv) -> h264 (h264_qsv))
Press [q] to stop, [?] for help
Output #0, h264, to 'test.h264':
  Metadata:
    encoder         : Lavf58.17.100
    Stream #0:0: Video: h264 (h264_qsv), qsv, 176x144, q=2-31, 1000 kb/s, 25 fps, 25 tbn, 25 tbc
    Metadata:
      encoder         : Lavc58.20.101 h264_qsv
    Side data:
      cpb: bitrate max/min/avg: 0/0/1000000 buffer size: 0 vbv_delay: -1
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
Error while decoding stream #0:0: Cannot allocate memory
[h264_qsv @ 0x42adf80] get_buffer() failed
[h264_qsv @ 0x42adf80] Too many errors when draining, this is a bug. Stop draining and force EOF.
Error while decoding stream #0:0: Internal bug, should not have happened
frame=   56 fps=0.0 q=-0.0 Lsize=     240kB time=00:00:02.52 bitrate= 780.0kbits/s speed=22.7x
video:240kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.000000%

[Zhong,Li]

Provide a patch 6434b84460bb8b996bc52b33b2012e6b05d3a459. Now can use the option extra_hw_frames to add the pool size just like other HWACCEL codecs such as vaapi/nvdec, though this is not a prefect solution.