increase default buffer size to 4 MB

2 MB was too small for cross-fading a 24 bit file.  Increasing to 4 MB
is still not too large for weak machines, but is enough for
cross-fading.

doc/mpdconf.example

@@ -382,7 +382,7 @@ input {
 # this may have undesired effects. Don't change this if you don't know what you
 # are doing.
 #
-#audio_buffer_size		"2048"
+#audio_buffer_size		"4096"
 #
 # This setting controls the percentage of the buffer which is filled before 
 # beginning to play. Increasing this reduces the chance of audio file skipping,