ffmpeg: re-add libpostproc as a source plugin

ffmpeg: re-add libpostproc as a source plugin

Upstream dropped libpostproc in 8.0; Kodi still requires it.

The --enable/--disable-postproc option the patch re-introduces is set
explicitly: on together with GPL in libffmpeg, off in libffmpeg-mini.

Patches:
  100-postproc: carried over from LibreELEC (Matthias Reichl's rebase
    of Michael Niedermayer's libpostproc source plugin;
    packages/multimedia/ffmpeg/patches/postproc/0001-postproc.patch as
    of LibreELEC commit 5f03a122f8): re-adds libpostproc as a source
    plugin

Header unchanged, hunks refreshed with quilt; the patched tree is
identical to the one LibreELEC's file produces.

Signed-off-by: Mirko Vogt <redacted>