ext/ffmpeg/: When doing QoS, don't drop the frame before decoding because we might drop an important reference frame,...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_base_init),
(gst_ffmpegdec_setcaps), (gst_ffmpegdec_video_frame):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_base_init):
When doing QoS, don't drop the frame before decoding because we might
drop an important reference frame, just make the decoder hurry_up on
this frame.