Move software mixer configuration out of audio driver code.

[AliceLR]

• Software mixer configuration is now performed by audio_mixer_init, which does basic sample rate checking and bounding on the buffer size and number of channels.
• The software mixer render function now takes a number of frames, channels, and the sample format instead of a number of bytes. It is up to the audio driver to calculate the number of frames needed if its API requests a number of bytes instead. The supported sample formats are SAMPLE_S16, SAMPLE_S8, and SAMPLE_U8.
• The software mixer now ignores audio streams with a NULL mix_data function instead of crashing. Nothing currently relies on this.
• The SDL audio driver will select whichever sample rate is returned by SDL_OpenAudioStream now instead of using the input value.