enable Theora in ffmpeg (again)

Hi,

I have a use case for Theora encoding with ffmpeg and would like to
bring this back to the port.

A brief history:

* ffmpeg's Theora support was removed in 2015, arguing that VP8/VP9 are
replacements. [1]
* ffmpeg2theora was removed in January 2019 because of breakage with
ffmpeg 4.x; claiming that "ffmpeg does the job nowadays" which in all
my trying has not been the case, with ffmpeg compiled without Theora
support. [2]

While this may be true for decoding, encoding Theora is now impossible
with the ffmpeg family. The FNA framework only uses Theora for video
playback because of the very compact implementation in
multimedia/libtheorafile:

$ du -hs /usr/local/lib/lib{theorafile,vpx}.so*
28.0K /usr/local/lib/libtheorafile.so.1.0
2.1M /usr/local/lib/libvpx.so.12.0

libvpx only contains support for VP8/9, but not VP3 (Theora).

I'm not sure what exactly the reasoning was for removing Theora support,
in the absence of a full replacement of this still-used codec. Below is
a diff to consider that would add a theora FLAVOR to ffmpeg. There is
no relevant difference in package size, so I'm not sure if this should
really be branched off into a flavor if it is considered:

23.2M ffmpeg-4.1.3p2v0-theora.tgz
23.2M ffmpeg-4.1.3p2v0.tgz

So, my question is if this diff could be considered, or clarification
if not.

[1] https://marc.info/?l=openbsd-ports-cvs&m=143020279411648&w=2
[2] https://marc.info/?l=openbsd-ports-cvs&m=154834827813518&w=2

Index: Makefile
===================================================================
RCS file: /cvs/ports/graphics/ffmpeg/Makefile,v
retrieving revision 1.184
diff -u -p -r1.184 Makefile
--- Makefile 23 May 2019 08:51:21 -0000 1.184
+++ Makefile 1 Jun 2019 17:37:44 -0000
@@ -4,7 +4,7 @@ COMMENT= audio/video converter and strea

V= 4.1.3
DISTNAME= ffmpeg-${V}
-REVISION= 1
+REVISION= 2
CATEGORIES= graphics multimedia
MASTER_SITES= https://ffmpeg.org/releases/
EXTRACT_SUFX= .tar.xz
@@ -162,8 +162,10 @@ VERSION_FLAGS= libavcodec_VERSION=${LIBa
libswresample_VERSION=${LIBswresample_VERSION} \
libswscale_VERSION=${LIBswscale_VERSION}

+AVCODEC_FLAGS= -lswresample -lavutil ${LIBavcodec_EXTRALIBS}
+
MAKE_FLAGS= ${VERSION_FLAGS} \
- LIBavcodec_EXTRALIBS="-lswresample -lavutil ${LIBavcodec_EXTRALIBS}" \
+ LIBavcodec_EXTRALIBS="${AVCODEC_FLAGS}" \
LIBavdevice_EXTRALIBS="-lavfilter -lswscale -lpostproc -lavformat -lavcodec -lswresample -lavresample -lavutil ${LIBavdevice_EXTRALIBS}" \
LIBavfilter_EXTRALIBS="-lswscale -lpostproc -lavformat -lavcodec -lswresample -lavresample -lavutil ${LIBavfilter_EXTRALIBS}" \
LIBavformat_EXTRALIBS="-lavcodec -lswresample -lavutil ${LIBavformat_EXTRALIBS}" \
@@ -174,6 +176,16 @@ MAKE_FLAGS= ${VERSION_FLAGS} \
LIBswscale_EXTRALIBS="-lavutil ${LIBswscale_EXTRALIBS}"
FAKE_FLAGS= ${VERSION_FLAGS} \
LDCONFIG=true
+
+FLAVORS = theora
+FLAVOR ?=
+
+.if ${FLAVOR:Mtheora}
+LIB_DEPENDS += multimedia/libtheora
+CONFIGURE_ARGS+= --enable-libtheora
+AVCODEC_FLAGS += -ltheora
+WANTLIB += theora
+.endif

.ifdef DEBUG
CONFIGURE_ARGS+=--disable-stripping