media-plugins/gst-plugins-libav: add ffmpeg-4 compatibility patches, other tweaks

media-plugins/gst-plugins-libav: add ffmpeg-4 compatibility patches, other tweaks

* Pull a selective patchset from git master, that brings in ffmpeg-4
  compatibility and some bug fixes; patchset tarball README has further details.
* Add a debian-inspired patch to tell gstreamer registry that the libav plugin
  supported features may change, when the system ffmpeg library files change.
  This should hopefully ensure that gstreamer sees new codecs immediately after
  system-ffmpeg is recompiled to add them. Compared to Debian, we conditionalize
  it based on USE=libav, so it's only actually done if system-ffmpeg is used, as
  for us it's a choice, not always system-ffmpeg; this is achieved via #ifndef,
  thus no conditional patching.
* Make bundled ffmpeg builds verbose for better build.log
* Try harder to honor user choices for bundled ffmpeg builds with USE=libav by
  always disabling debug and passing CFLAGS as optflags; courtesy of
  Arfrever Frehtes Taifersar Arahesis

Closes: https://bugs.gentoo.org/654628
Signed-off-by: Mart Raudsepp <leio@gentoo.org>
Package-Manager: Portage-2.3.52, Repoman-2.3.11