Fails to build with ffmpeg 3.0

[yurivict]

  movie.cc:56:42: error: use of undeclared identifier 'PIX_FMT_RGB24'; did you mean
    'AV_PIX_FMT_RGB24'?
              width_, height_, PIX_FMT_RGB24, SWS_FAST_BILINEAR,
                       ^~~~~~~~~~~~~
                       AV_PIX_FMT_RGB24
  movie.cc:64:18: error: use of undeclared identifier 'avcodec_alloc_frame'
    CHECK(frame_ = avcodec_alloc_frame());
           ^

Possible fix: s/PIX_FMT/AV_&/ + s/avcodec_alloc_frame/av_frame_alloc/

[jart]

This is fixed in #27 but the problem is the PR needs to be modified to maintain compatibility with Ubuntu 14.04. So hopefully @tombh will be kind enough to add the necessary define statements so we can get that merged.

[citrus-lemon]

this problem also occurs in ffmpeg 4.0+
I write the issue
#53 and pull requests #52