"Underrun occurred" warnings when using ALSA audio driver

[thwill1000]

When using the ALSA audio driver, i.e.

export SDL_AUDIODRIVER=alsa

warnings of this form are written to the console:

ALSA lib pcm.c:8526:(snd_pcm_recover) underrun occurred
ALSA lib pcm.c:8526:(snd_pcm_recover) underrun occurred
ALSA lib pcm.c:8526:(snd_pcm_recover) underrun occurred
ALSA lib pcm.c:8526:(snd_pcm_recover) underrun occurred
ALSA lib pcm.c:8526:(snd_pcm_recover) underrun occurred
ALSA lib pcm.c:8526:(snd_pcm_recover) underrun occurred

I believe this is occurring because the driver is requesting audio samples faster than the MMB4L code is providing them.

I think the solution to this problem is to increase the number of samples in audio_configure() (there is a TODO in the code to this effect) and make corresponding changes to audio_callback() and its sub-functions.

Reported by @Volhout, but also seen and put on the back-burner during original development.