avutil/frame: add a keyframe flag to AVFrame

Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/frame.h

@@ -590,6 +590,10 @@ typedef struct AVFrame {
  * The frame data may be corrupted, e.g. due to decoding errors.
  */
 #define AV_FRAME_FLAG_CORRUPT       (1 << 0)
+/**
+ * A flag to mark frames that are keyframes.
+ */
+#define AV_FRAME_FLAG_KEY (1 << 1)
 /**
  * A flag to mark the frames which need to be decoded, but shouldn't be output.
  */