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 <mirko-openwrt@nanl.de>

multimedia/ffmpeg/Config.in

@@ -3,12 +3,13 @@ menu "FFmpeg configuration"
 comment "Licensing"
 
 config FFMPEG_GPL
-	bool "Include GPL components (GPL filters)"
+	bool "Include GPL components (libpostproc, GPL filters)"
 	default y
 	help
-	  Build libffmpeg with --enable-gpl. This adds the
-	  GPL-licensed filters, and the resulting libraries are
-	  distributed under the GPL-2.0-or-later.
+	  Build libffmpeg with --enable-gpl. This adds libpostproc
+	  and the GPL-licensed filters, and the resulting libraries are
+	  distributed under the GPL-2.0-or-later. Kodi requires
+	  libpostproc.
 
 config FFMPEG_VERSION3
 	bool "Allow (L)GPL v3 components"

multimedia/ffmpeg/Makefile

@@ -487,12 +487,17 @@ FFMPEG_OPT=$(if $(CONFIG_$(1)),--enable-$(2),--disable-$(2))
 
 ifeq ($(BUILD_VARIANT),default)
 
+  # postproc: libpostproc was removed upstream in FFmpeg 8.0 and is re-added
+  # by patches/100-postproc.patch (LibreELEC), which also re-introduces the
+  # --enable/--disable-postproc option; it is GPL-only, so it follows the GPL
+  # option. Kodi requires it.
   # vaapi/libdav1d: literal --disable until the libva / dav1d packages exist
   # in the feeds; then restore $(call FFMPEG_OPT,FFMPEG_VAAPI,vaapi),
   # $(call FFMPEG_OPT,FFMPEG_LIBDAV1D,libdav1d) and, for manual component
   # selection, --enable-hwaccel=*_vaapi / --enable-decoder=libdav1d.
   FFMPEG_CONFIGURE+= \
 	$(call FFMPEG_OPT,FFMPEG_GPL,gpl) \
+	$(call FFMPEG_OPT,FFMPEG_GPL,postproc) \
 	$(call FFMPEG_OPT,FFMPEG_VERSION3,version3) \
 	$(call FFMPEG_OPT,FFMPEG_NONFREE,nonfree) \
 	$(call FFMPEG_OPT,FFMPEG_OPTIMIZE_SIZE,small) \
@@ -577,6 +582,7 @@ ifeq ($(BUILD_VARIANT),mini)
 	--disable-programs \
 	--disable-avdevice \
 	--disable-avfilter \
+	--disable-postproc \
 	--disable-swresample \
 	--disable-swscale \
 	--disable-everything \