Log Message:
gstreamer1 + plugins: update to 1.12.4

### 1.12.4

The fourth 1.12 bug-fix release (1.12.4) was released on 7 December 2017.
This release only contains bugfixes and it should be safe to update from 1.12.x.

#### Major bugfixes in 1.12.4

 - Dozens of fixes for various issues detected with the help of Google's OSS-Fuzz
   project: https://github.com/google/oss-fuzz
   Details to be found there in the bug tracker
 - Performance regressions with registering debug categories after gst_init()
   were fixed
 - Regression with seeking back to 0 in souphttpsrc was fixed
 - Regression with header rewriting in flacparse was fixed
 - Regression with playbin/playsink leaking sinks was fixed
 - Inconsistencies with DROPPED/HANDLED handling in pad probe handlers are fixed
 - gst_bin_iterate_sorted() always returns sources last now, as documented
 - gst_query_writable_structure() will never return NULL, for consistency with events
 - Removal of metas from gst_buffer_meta_foreach() works correctly now
 - OpenJPEG plugin builds with OpenJPEG >= 2.3
 - CDIO plugin builds with CDIO >= 1.0
 - gstreamer-vaapi works correctly with libva 1.0
 - gst-libav was updated to ffmpeg 3.3.5
 - Various fixes for memory leaks, deadlocks and crashes in all modules
 - ... and many, many more!