upgrade source to 2.0.8

Edward Rudd

2018-08-19 561f0d614098a95527367cc3f911e476f35643d6

 source/Xcode-iOS/Demos/src/mixer.c

@@ -250,7 +250,7 @@

        /* mix this sound effect with the output */
        SDL_MixAudioFormat(stream, mixer.channels[i].position,
                           mixer.outputSpec.format, copy_amt, 150);
                           mixer.outputSpec.format, copy_amt, SDL_MIX_MAXVOLUME);

        /* update buffer position in sound effect and the number of bytes left */
        mixer.channels[i].position += copy_amt;