ext/ffmpeg/gstffmpegdec.c: Improve pixel aspect ratio handling when both the demuxer and the decoder provide a pixel ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_close),
(gst_ffmpegdec_setcaps), (gst_ffmpegdec_add_pixel_aspect_ratio),
(gst_ffmpegdec_negotiate):
Improve pixel aspect ratio handling when both the demuxer and the
decoder provide a pixel aspect ratio: if one of the two ratios is
1:1, use the other one; if both are non-1:1, use the one provided
by the decoder (fixes matroska starwars test files); also add some
locking here and there.