Add FM synthesizer support

This adds support for "real" FM sound that can be used instead of MIDI
sound. (Currently only available on platforms where SDL_mixer is used.)

The MakoFM class interprets AMUS.DAT to create a write sequence to the
OPNA registers, from which fmgen generates an audio stream.

.github/workflows/linux.yml

@@ -11,6 +11,8 @@ jobs:
     name: Linux ${{ matrix.build-type }}
     steps:
     - uses: actions/checkout@v2
+      with:
+        submodules: true
 
     - name: Install Deps
       run: |