vlc_es: add HDR10+ dynamic metadata

Following the precedent set by the DoVi metadata, we encode this as an
ancillary attached directly to the frame, since it contains
frame-dynamic data.

We make the decision to follow ATSC A/341 Amendment 2094-40 instead of
the original SMPTE ST2094-40:2016 specification, because the latter
includes a lot of extra metadata fields which the former intentionally
leaves unused, and which are also unused in practice (both in
implementation and in files). We can easily add them later should a need
arise.

Also, again following the precedent set by `vlc_video_dovi_metadata_t`,
we encode AVRationals directly as `float`. This saves us from having to
do another unnecessary conversion step in the actual usage site (e.g.
the libplacebo vout) by allowing the use of `av_q2d` directly.

include/vlc_es.h

@@ -610,6 +610,39 @@ typedef struct vlc_video_dovi_metadata_t
     } nlq[3];
 } vlc_video_dovi_metadata_t;
 
+/**
+ * HDR10+ Dynamic metadata (based on ATSC A/341 Amendment 2094-40)
+ *
+ * This is similar to SMPTE ST2094-40:2016, but omits the mastering display and
+ * target display actual peak luminance LUTs, the rectangular boundaries and
+ * ellipse coefficients, and support for multiple processing windows, as these
+ * are intentionally left unused in this version of the specification.
+ */
+
+#define VLC_ANCILLARY_ID_HDR10PLUS VLC_FOURCC('H','D','R','+')
+
+typedef struct vlc_video_hdr_dynamic_metadata_t
+{
+    uint8_t country_code;           /* ITU-T T.35 Annex A */
+    uint8_t application_version;
+    float targeted_luminance;       /* in cd/m² */
+
+    /* parameters for the first processing window (encompassing the frame) */
+    float maxscl[3];                /* in linearized range [0,1] */
+    float average_maxrgb;           /* in linearized range [0,1] */
+    uint8_t num_histogram;          /* in range [0,15] */
+    struct {
+        uint8_t percentage;         /* in range [1,100] */
+        float percentile;           /* in linearized range [0,1] */
+    } histogram[15];
+    float fraction_bright_pixels;/* in range [0,1] */
+    uint8_t tone_mapping_flag;
+    float knee_point_x;             /* in ootf range [0,1] */
+    float knee_point_y;             /* in ootf range [0,1] */
+    uint8_t num_bezier_anchors;     /* in range [1,15] */
+    float bezier_curve_anchors[15]; /* in range [0,1] */
+} vlc_video_hdr_dynamic_metadata_t;
+
 /**
  * Embedded ICC profiles
  */