set SDLMIXER_VENDORED to ON and vendor the required libraries into libs/SDL3_mixer/external

disable SDLMIXER_VORBIS_VORBISFILE
add ogg v1.3.5-SDL
add opus v1.4.x-SDL
add opusfile v0.13-git-SDL

libs/SDL3_mixer/external/opusfile/doc/Makefile

@@ -0,0 +1,35 @@
+## GNU makefile for opusfile documentation.
+
+-include ../package_version
+
+all: doxygen
+
+doxygen: Doxyfile ../include/opusfile.h
+	doxygen
+
+pdf: doxygen
+	make -C latex
+
+clean:
+	$(RM) -r html
+	$(RM) -r latex
+
+distclean: clean
+	$(RM) Doxyfile
+
+.PHONY: all clean distclean doxygen pdf
+
+../package_version:
+	@if [ -x ../update_version ]; then \
+		../update_version || true; \
+	elif [ ! -e $@ ]; then \
+		echo 'PACKAGE_VERSION="unknown"' > $@; \
+	fi
+
+# run autoconf-like replacements to finalize our config
+Doxyfile: Doxyfile.in Makefile ../package_version
+	sed -e 's/@PACKAGE_NAME@/opusfile/' \
+	    -e 's/@PACKAGE_VERSION@/$(PACKAGE_VERSION)/' \
+	    -e 's/@HAVE_DOT@/yes/' \
+	    -e 's/@top_srcdir@/../' \
+	  < $< > $@