avframe: mark source frame const in _ref and _clone

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/frame.c

@@ -171,7 +171,7 @@ int av_frame_get_buffer(AVFrame *frame, int align)
     return AVERROR(EINVAL);
 }
 
-int av_frame_ref(AVFrame *dst, AVFrame *src)
+int av_frame_ref(AVFrame *dst, const AVFrame *src)
 {
     int i, ret = 0;
 
@@ -257,7 +257,7 @@ fail:
     return ret;
 }
 
-AVFrame *av_frame_clone(AVFrame *src)
+AVFrame *av_frame_clone(const AVFrame *src)
 {
     AVFrame *ret = av_frame_alloc();
 

libavutil/frame.h

@@ -384,7 +384,7 @@ void av_frame_free(AVFrame **frame);
  *
  * @return 0 on success, a negative AVERROR on error
  */
-int av_frame_ref(AVFrame *dst, AVFrame *src);
+int av_frame_ref(AVFrame *dst, const AVFrame *src);
 
 /**
  * Create a new frame that references the same data as src.
@@ -393,7 +393,7 @@ int av_frame_ref(AVFrame *dst, AVFrame *src);
  *
  * @return newly created AVFrame on success, NULL on error.
  */
-AVFrame *av_frame_clone(AVFrame *src);
+AVFrame *av_frame_clone(const AVFrame *src);
 
 /**
  * Unreference all the buffers referenced by frame and reset the frame fields.