ext/ffmpeg/gstffmpegdec.c: Reenable pad_alloc, seem to work now.

ext/ffmpeg/gstffmpegdec.c: Reenable pad_alloc, seem to work now.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_class_init),
(gst_ffmpegdec_init), (gst_ffmpegdec_close), (gst_ffmpegdec_open),
(gst_ffmpegdec_setcaps), (gst_ffmpegdec_get_buffer),
(gst_ffmpegdec_release_buffer), (get_output_buffer),
(gst_ffmpegdec_video_frame), (gst_ffmpegdec_audio_frame),
(gst_ffmpegdec_frame), (gst_ffmpegdec_change_state),
(gst_ffmpegdec_set_property), (gst_ffmpegdec_get_property):
Reenable pad_alloc, seem to work now.
Added property to easily disable it later on.
Remove some old code that tried hard to break the get_buffer
functions. Fixes #321662.