Fix SDL audio delay

Fixes #647

300ms is just too big, and produces an audible delay. 100ms sounds
much better.

src/common/SoundSDL.cpp

@@ -25,8 +25,8 @@
 
 extern int emulating;
 
-// Hold up to 300 ms of data in the ring buffer
-const double SoundSDL::buftime = 0.300;
+// Hold up to 100 ms of data in the ring buffer
+const double SoundSDL::buftime = 0.100;
 
 SoundSDL::SoundSDL():
     samples_buf(0),