ext/ffmpeg/gstffmpegdec.c: Change the pad_alloc calculations for weird clipped sizes, refactor the code a bit.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_setcaps),
(alloc_output_buffer), (gst_ffmpegdec_get_buffer),
(gst_ffmpegdec_release_buffer), (gst_ffmpegdec_negotiate),
(get_output_buffer):
Change the pad_alloc calculations for weird clipped sizes, refactor the
code a bit.
Add support for some different refcounting algorithm.
Direct rendering still disabled by default.