ggwave : add "soundMarkerThreshold" parameter

Can be used to control the threshold used for distinguishing odd from even
frequencies in the sound markers.

examples/ggwave-common-sdl2.cpp

@@ -220,6 +220,7 @@ bool GGWave_init(
             g_obtainedSpecInp.freq,
             g_obtainedSpecOut.freq,
             GGWave::kDefaultSamplesPerFrame,
+            GGWave::kDefaultSoundMarkerThreshold,
             sampleFormatInp,
             sampleFormatOut});
     }