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/release.md

@@ -0,0 +1,58 @@
+# Release checklist
+
+## Source release
+
+- Update OP_LT_* API versioning in configure.ac.
+- Check for uncommitted changes to master.
+- Prepare win32 binaries
+  - Do this before tagging the release, as it may require changes which should
+    be committed
+- Tag the release commit with 'git tag -s vN.M'.
+  - Include release notes in the tag annotation.
+- Verify 'make distcheck' produces a tarball with
+  the desired name.
+- Push tag to public repo.
+- Upload source package 'opusfile-${version}.tar.gz'
+  to website and verify file permissions.
+- Update checksum files on website.
+- Update links on <https://www.opus-codec.org/downloads/>.
+- Add a copy of the documentation to <https://www.opus-codec.org/docs/>
+  and update the links.
+  - Add doc/latex/refman as docs/opusfile_api-${version}.pdf on opus-codec.org
+  - Add doc/html as docs/opusfile_api-${version} on opus-codec.org
+
+Releases are commited to https://svn.xiph.org/releases/opus/
+which propagates to downloads.xiph.org, and copied manually
+to https://archive.mozilla.org/pub/opus/
+
+Release notes and package links should be added to the corresponding
+tag at https://gitlab.xiph.org/xiph/opusfile so they show on the
+releases page.
+
+Release packages should also be manually attached to the corresponding
+tag on the github mirror https://github.com/xiph/opusfile/releases
+
+## Win32 binaries
+
+- Install cross-i686-w64-mingw32-gcc and associated binutils.
+  - If you skip this step, libopus will still try to build with the system gcc
+    and then fail to link.
+- Edit mingw/Makefile to point to the latest versions of libogg. opus, openssl
+  (see <https://archive.mozilla.org/pub/opus/>, checksums in SHA256SUMS.txt)
+- run `make -C mingw`
+  - Downloads versions of libogg, opus, openssl.
+  - Compiles them.
+  - Compiles static opusfile and examples against the built deps.
+- run `make -C mingw package`
+  - Creates an opusfile-${version}-win32.zip binary package.
+- Merge changes between README.md and the version in the last
+  binary release. E.g. it's good to include versions of the dependencies,
+  release notes, etc.
+- Copy the archive to a clean system and verify the examples work
+  to make sure you've included all the necessary libraries.
+- Upload the archive zipfile to websites.
+- Verify file permissions and that it's available at the expected URL.
+- Update links on <https://www.opus-codec.org/downloads/>.
+
+Binary releases are copied manually to s3 to appear at
+https://archive.mozilla.org/pub/mozilla.org/opus/win32/