ext/ffmpeg/: Don't use GINT_TO_POINTER and GPOINTER_TO_INT with GTypes, this can break horribly if sizeof(GType) happ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_base_init),
(gst_ffmpegdec_register):
* ext/ffmpeg/gstffmpegdemux.c: (gst_ffmpegdemux_base_init),
(gst_ffmpegdemux_register):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_base_init),
(gst_ffmpegenc_register):
* ext/ffmpeg/gstffmpegmux.c: (gst_ffmpegmux_base_init),
(gst_ffmpegmux_register):
Don't use GINT_TO_POINTER and GPOINTER_TO_INT with GTypes, this
can break horribly if sizeof(GType) happens to be bigger than
sizeof(int), because GPOINTER_TO_INT might then chop off some bits
of our GType (the reason this seems to works nevertheless is the
put-current-type-also-as-value-0-into-the-hash-table-as-fallback
hack used in the current code).
In any case, instead of just fixing this, let's not use a hash table
with GTypes here at all. g_type_{set|get}_qdata() seems to do the
job just as well.