Fix possible Vorbis buffer underruns.

There're two possible problems with the calculation of the number of
sound buffers for Vorbis if OpenAL is in use:

* We assume that the (more or less) maximum number buffers is allocated
  during map load. This is not correct if in a multiplayer game a lot of
  custom models with custom sound connect at a later time.
* 64 buffers (about 3 seconds worth of music) may be too low in some
  situations.

Work around this by recalculating the number of buffers if necessary.
We're now reserving about 256 (== 12 seconds) buffers.

This may fix issue #252.

src/client/sound/ogg.c

@@ -636,18 +636,12 @@ OGG_Stream(void)
 			/* Calculate the number of buffers used
 			   for storing decoded OGG/Vorbis data.
 			   We take the number of active buffers
-			   at startup (at this point most of the
-			   samples should be precached and loaded
-			   into buffers) and add 64. Empircal
-			   testing showed, that at most times
-			   at least 52 buffers remain available
-			   for OGG/Vorbis, enough for about 3
-			   seconds playback. The music won't
-			   stutter as long as the framerate
-			   stayes over 1 FPS. */
-			if (ogg_numbufs == 0)
+			   and add 256. 256 are about 12 seconds
+			   worth of sound, more than enough to
+			   be resilent against underruns. */
+			if (ogg_numbufs == 0 || active_buffers < ogg_numbufs - 256)
 			{
-				ogg_numbufs = active_buffers + 64;
+				ogg_numbufs = active_buffers + 256;
 			}
 
 			/* active_buffers are all active OpenAL buffers,