ext/ffmpeg/gstffmpegdec.c: Implement B-frame skipping and low-resolution decoding (useful for very slow CPUs).

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_lowres_get_type),
(gst_ffmpegdec_skipframe_get_type), (gst_ffmpegdec_class_init),
(gst_ffmpegdec_init), (gst_ffmpegdec_connect),
(gst_ffmpegdec_frame), (gst_ffmpegdec_set_property),
(gst_ffmpegdec_get_property):
Implement B-frame skipping and low-resolution decoding (useful
for very slow CPUs).