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/opus/.travis.yml

@@ -0,0 +1,21 @@
+language: c
+
+compiler:
+  - gcc
+  - clang
+
+os:
+  - linux
+  - osx
+
+env:
+  - CONFIG=""
+  - CONFIG="--enable-assertions"
+  - CONFIG="--enable-fixed-point"
+  - CONFIG="--enable-fixed-point --disable-float-api"
+  - CONFIG="--enable-fixed-point --enable-assertions"
+
+script:
+  - ./autogen.sh
+  - ./configure $CONFIG
+  - make distcheck