ao_alsa: reduce default buffer size

In general, we don't need to have a large hw audio buffer size anymore,
because we can quickly fill it from the soft buffer.

Note that this probably doesn't change much anyway. On my system (dmix
enabled), the buffer size is only 170ms, and ALSA won't give more. Even
when using a hardware device the buffer size seems to be limited to
341ms.

audio/out/ao_alsa.c

@@ -67,7 +67,7 @@ struct priv {
     int cfg_resample;
 };
 
-#define BUFFER_TIME 500000  // 0.5 s
+#define BUFFER_TIME 250000  // 250ms
 #define FRAGCOUNT 16
 
 #define CHECK_ALSA_ERROR(message) \