multimedia/py-ffmpeg: adjust pts after r425471

AVFrame.pkt_pts didn't exist in ffmpeg-4f7d2fe-2010-12-16 snapshot when
global_video_pkt_pts was added to py-ffmpeg. The code worked it around by
saving PTS in AVFrame.opaque just before avcodec_decode_video2() call. This
broke when r425471 removed our_get_buffer(). So, just use AVFrame.pkt_pts
directly now.

PR: 214247
Approved by: dbn (maintainer)