avutil/frame: add a new side data type for downmix matrix

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

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2026-06-23.
 
 API changes, most recent first:
 
+2026-07-11 - xxxxxxxxxxx - lavu 61.4.100 - frame.h
+  Add AV_FRAME_DATA_DOWNMIX_MATRIX.
+
 2026-07-11 - xxxxxxxxxxx - lavu 61.3.100 - downmix_info.h
   Add AVDownmixMatrix, AVDownmixCoeff.
   Add av_downmix_matrix_coeff(), av_downmix_matrix_alloc().

libavutil/frame.h

@@ -299,6 +299,12 @@ enum AVFrameSideDataType {
      * libavutil/raw_color_params.h.
      */
     AV_FRAME_DATA_RAW_COLOR_PARAMS,
+
+    /**
+     * Metadata relevant to a downmix procedure in the form of a remixig matrix.
+     * The data is the AVDownmixMatrix struct defined in libavutil/downmix_info.h.
+     */
+    AV_FRAME_DATA_DOWNMIX_MATRIX,
 };
 
 enum AVActiveFormatDescription {

libavutil/side_data.c

@@ -62,6 +62,7 @@ static const AVSideDataDescriptor sd_props[] = {
     [AV_FRAME_DATA_IAMF_DEMIXING_INFO_PARAM]    = { "IAMF Demixing Info Parameter Data",            AV_SIDE_DATA_PROP_CHANNEL_DEPENDENT },
     [AV_FRAME_DATA_IAMF_RECON_GAIN_INFO_PARAM]  = { "IAMF Recon Gain Info Parameter Data" },
     [AV_FRAME_DATA_RAW_COLOR_PARAMS]            = { "RAW camera color parameters",                  AV_SIDE_DATA_PROP_GLOBAL | AV_SIDE_DATA_PROP_COLOR_DEPENDENT },
+    [AV_FRAME_DATA_DOWNMIX_MATRIX]              = { "Downmix Matrix",                               AV_SIDE_DATA_PROP_CHANNEL_DEPENDENT },
 };
 
 const AVSideDataDescriptor *av_frame_side_data_desc(enum AVFrameSideDataType type)

libavutil/version.h

@@ -79,7 +79,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR  61
-#define LIBAVUTIL_VERSION_MINOR   3
+#define LIBAVUTIL_VERSION_MINOR   4
 #define LIBAVUTIL_VERSION_MICRO 100
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \