video/image_writer: avoid stripping colorspace info when writing image

Writing an image either with vo_image or with a screenshot will strip
the colorspace info because it allocates a new mp_image that contains
the same data as the old image after calling mp_image_params_guess_csp.
However, mp_image_params_guess_csp cannot always guess the appropriate
colorspace, so it picks a "sane default." Since this function also
changes parameters so the space always makes sense, this extra info
isn't harmful and allows screenshots and vo_image outs to be properly
tagged with the correct colorspace.

Fixes #10988.

video/image_writer.c

@@ -355,6 +355,7 @@ static struct mp_image *convert_image(struct mp_image *image, int destfmt,
         .h = d_h,
         .p_w = 1,
         .p_h = 1,
+        .color = image->params.color,
     };
     mp_image_params_guess_csp(&p);