Add a get_sbits_long() function.

Originally committed as revision 17740 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/bitstream.h

@@ -33,6 +33,7 @@
 #include "libavutil/common.h"
 #include "libavutil/intreadwrite.h"
 #include "libavutil/log.h"
+#include "mathops.h"
 
 #if defined(ALT_BITSTREAM_READER_LE) && !defined(ALT_BITSTREAM_READER)
 #   define ALT_BITSTREAM_READER
@@ -706,6 +707,13 @@ static inline unsigned int get_bits_long(GetBitContext *s, int n){
     }
 }
 
+/**
+ * reads 0-32 bits as a signed integer.
+ */
+static inline int get_sbits_long(GetBitContext *s, int n) {
+    return sign_extend(get_bits_long(s, n), n);
+}
+
 /**
  * shows 0-32 bits.
  */