libsaria: Improve random_next()

Limiting rand to plist.size() * (3 / 4) kept causing songs at the end of
the range to get selected.  This caused a backwards-scrolling effect as
songs were played.  Instead, I want to pick from the next half of the
list to create a forward-scrolling effect.

Signed-off-by: Bryan Schumaker <bjschuma@gmail.com>

libsaria/playlist/next.cpp

@@ -62,7 +62,7 @@ namespace libsaria
 			return picked_next();
 		}
 
-		n = rand() % ((size * 3) / 4);
+		n = rand() % (size / 2);
 		for (unsigned int i = 0; i < n; i++)
 			incr_iter();