Update public FFmpeg header files from r16647 to r18286.

third_party/ffmpeg/include/libavutil/mathematics.h

Index: third_party/ffmpeg/include/libavutil/mathematics.h
diff --git a/third_party/ffmpeg/include/libavutil/mathematics.h b/third_party/ffmpeg/include/libavutil/mathematics.h
index 74385274b30a93432d9f6320cf21e05d35a38097..782efcca0f176d91ca6f70f38debdf09dc163c35 100644
--- a/third_party/ffmpeg/include/libavutil/mathematics.h
+++ b/third_party/ffmpeg/include/libavutil/mathematics.h
@@ -23,6 +23,7 @@
 
 #include <stdint.h>
 #include <math.h>
+#include "common.h"
 #include "rational.h"
 
 #ifndef M_E
@@ -42,27 +43,29 @@
 #endif
 
 enum AVRounding {
-    AV_ROUND_ZERO     = 0, ///< round toward zero
-    AV_ROUND_INF      = 1, ///< round away from zero
-    AV_ROUND_DOWN     = 2, ///< round toward -infinity
-    AV_ROUND_UP       = 3, ///< round toward +infinity
-    AV_ROUND_NEAR_INF = 5, ///< round to nearest and halfway cases away from zero
+    AV_ROUND_ZERO     = 0, ///< Round toward zero.
+    AV_ROUND_INF      = 1, ///< Round away from zero.
+    AV_ROUND_DOWN     = 2, ///< Round toward -infinity.
+    AV_ROUND_UP       = 3, ///< Round toward +infinity.
+    AV_ROUND_NEAR_INF = 5, ///< Round to nearest and halfway cases away from zero.
 };
 
+int64_t av_const av_gcd(int64_t a, int64_t b);
+
 /**
- * rescale a 64bit integer with rounding to nearest.
- * a simple a*b/c isn't possible as it can overflow
+ * Rescales a 64-bit integer with rounding to nearest.
+ * A simple a*b/c isn't possible as it can overflow.
  */
 int64_t av_rescale(int64_t a, int64_t b, int64_t c) av_const;
 
 /**
- * rescale a 64bit integer with specified rounding.
- * a simple a*b/c isn't possible as it can overflow
+ * Rescales a 64-bit integer with specified rounding.
+ * A simple a*b/c isn't possible as it can overflow.
  */
 int64_t av_rescale_rnd(int64_t a, int64_t b, int64_t c, enum AVRounding) av_const;
 
 /**
- * rescale a 64bit integer by 2 rational numbers.
+ * Rescales a 64-bit integer by 2 rational numbers.
  */
 int64_t av_rescale_q(int64_t a, AVRational bq, AVRational cq) av_const;