ac3enc: Change EXP_DIFF_THRESHOLD to 500.

This patch changes the exponent difference threshold in the exponent
strategy decision function of the AC-3 encoder.  I tested lowering in
increments of 100.  From 1000 down to 500 generally increased in quality
with each step, but 400 was generally much worse.
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit c3beafa0)

libavcodec/ac3enc.c

@@ -434,7 +434,7 @@ static void extract_exponents(AC3EncodeContext *s)
  * Exponent Difference Threshold.
  * New exponents are sent if their SAD exceed this number.
  */
-#define EXP_DIFF_THRESHOLD 1000
+#define EXP_DIFF_THRESHOLD 500
 
 
 /**

tests/ref/acodec/ac3_fixed

-9823c8f74097eab5d148cf0536ae932e *./tests/data/acodec/ac3.rm
+37e119c644ba0472c079d0e907bf78df *./tests/data/acodec/ac3.rm
 98751 ./tests/data/acodec/ac3.rm

tests/ref/lavf/rm

-e1990def7b195a568340f2c1a89d6587 *./tests/data/lavf/lavf.rm
+b97899ef2812cd1c8851dea550e042e4 *./tests/data/lavf/lavf.rm
 346706 ./tests/data/lavf/lavf.rm