lavu/frame: allow calling av_frame_make_writable() on non-refcounted frames

This is an easy way to make a refcounted frame from a non-refcounted
one.

doc/APIchanges

@@ -14,6 +14,11 @@ libavutil:     2021-04-27
 
 API changes, most recent first:
 
+2022-08-xx - xxxxxxxxxx - lavu 57.31.100 - frame.h
+  av_frame_make_writable() may now be called on non-refcounted
+  frames and will make a refcounted copy out of them.
+  Previously an error was returned in such cases.
+
 2022-07-xx - xxxxxxxxx - lavc 59.40.100 - avcodec.h
   Add the AV_CODEC_FLAG2_ICC_PROFILES flag to AVCodecContext, to enable
   automatic reading and writing of embedded ICC profiles in image files.

libavutil/frame.c

@@ -552,9 +552,6 @@ int av_frame_make_writable(AVFrame *frame)
     AVFrame tmp;
     int ret;
 
-    if (!frame->buf[0])
-        return AVERROR(EINVAL);
-
     if (av_frame_is_writable(frame))
         return 0;
 

libavutil/frame.h

@@ -828,7 +828,8 @@ int av_frame_is_writable(AVFrame *frame);
  * Ensure that the frame data is writable, avoiding data copy if possible.
  *
  * Do nothing if the frame is writable, allocate new buffers and copy the data
- * if it is not.
+ * if it is not. Non-refcounted frames behave as non-writable, i.e. a copy
+ * is always made.
  *
  * @return 0 on success, a negative AVERROR on error.
  *

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  57
-#define LIBAVUTIL_VERSION_MINOR  30
+#define LIBAVUTIL_VERSION_MINOR  31
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \