video/image_writer: tag bits_per_raw_sample when pixfmt changes

This populates the AVCodecContext->bits_per_raw_sample field value when
the pixel format of the screenshot being written doesn't match the pixel format
of the original video. For example, if the original video was 10-bit and the
screenshot is 16-bit, it populates this field so the encoder can make use of it.

Currently only the libjxl encoder respects this tag although PNG supports sBIT.
Once lavc's PNG encoder respects this tag it will work seamlessly with this commit
as well.

This feature is only enabled if screenshot-high-bit-depth=yes is enabled, because
bits_per_raw_sample shouldn't be set for 8-bit screenshots.

video/image_writer.c

@@ -22,6 +22,7 @@
 #include <libavcodec/avcodec.h>
 #include <libavutil/mem.h>
 #include <libavutil/opt.h>
+#include <libavutil/pixdesc.h>
 
 #include "config.h"
 
@@ -128,6 +129,24 @@ static bool write_lavc(struct image_writer_ctx *ctx, mp_image_t *image, FILE *fp
     avctx->height = image->h;
     avctx->color_range = mp_csp_levels_to_avcol_range(image->params.color.levels);
     avctx->pix_fmt = imgfmt2pixfmt(image->imgfmt);
+
+    /*
+     * tagging avctx->bits_per_raw_sample indicates the number of significant
+     * bits. For example, if the original video was 10-bit, and the GPU buffer is
+     * 16-bit, this tells lavc that only 10 bits are significant. lavc encoders may
+     * ignore this value, but some codecs can make use of it (for example, PNG's
+     * sBIT chunk or JXL's bit depth header)
+     */
+    if (ctx->opts->high_bit_depth && ctx->original_format.id != image->imgfmt) {
+        const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(imgfmt2pixfmt(ctx->original_format.id));
+        int depth = 0;
+        for (int i = 0; i < desc->nb_components; i++) {
+            if (desc->comp[i].depth > depth)
+                depth = desc->comp[i].depth;
+        }
+        avctx->bits_per_raw_sample = depth;
+    }
+
     if (codec->id == AV_CODEC_ID_MJPEG) {
         // Annoying deprecated garbage for the jpg encoder.
         if (image->params.color.levels == MP_CSP_LEVELS_PC)