ext/ffmpeg/: Add mappings for RealVideo 4.0, AMR-WB, AMR-NB and DTS to silence warnings when registering.

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcodecmap.c: (gst_ffmpeg_codecid_to_caps),
(gst_ffmpeg_caps_with_codecid), (gst_ffmpeg_formatid_get_codecids),
(gst_ffmpeg_caps_to_codecid), (gst_ffmpeg_get_codecid_longname):
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_class_init):
Add mappings for RealVideo 4.0, AMR-WB, AMR-NB and DTS to silence
warnings when registering.
Only register the 1/2,1/4 size decode and frame-skipping properties
on video decoders.
* gst-libs/ext/ffmpeg/configure.ac:
* gst-libs/ext/ffmpeg/libavformat/Makefile.am:
* gst-libs/ext/ffmpeg/patches/autotools.patch:
Merge patch by Sam Morris to allow disabling of decoders.
Fixes #324366