Tag problematic DNG image files

As commented in commit cf9d9f6 there a dng files still problematic for darktable.
While reading exif data they now automatically are tagged with "darktable|github|dng_issue"
so they can be found easily by the user.

Author: jenshannoschwalm

src/common/exif.cc

@@ -1781,6 +1781,8 @@ static bool _exif_decode_exif_data(dt_image_t *img, Exiv2::ExifData &exifData)
     // Read embedded color matrix and DNG related tags.
     if(FIND_EXIF_TAG("Exif.Image.DNGVersion"))
     {
+      gboolean github_request = FALSE;
+
       // initialize matrixes and data with noop / defaults
       float CM[3][9];
       float FM[3][9];
@@ -1918,6 +1920,7 @@ static bool _exif_decode_exif_data(dt_image_t *img, Exiv2::ExifData &exifData)
           dt_control_log(_("detected OtherIlluminant in `%s`, please report via darktable github"), img->filename);
           dt_print(DT_DEBUG_IMAGEIO, "detected not-implemented OtherIlluminant in `%s`, please report via darktable github", img->filename);
           illu[k] = DT_LS_D65;
+          github_request = TRUE;
         }
 
         if(illu[k] != DT_LS_Unknown)
@@ -1965,6 +1968,7 @@ static bool _exif_decode_exif_data(dt_image_t *img, Exiv2::ExifData &exifData)
       {
         dt_control_log(_("special exif illuminants in `%s`, please report via darktable github"), img->filename);
         dt_print(DT_DEBUG_ALWAYS, "special exif illuminants in `%s`, please report via darktable github", img->filename);
+        github_request = TRUE;
       }
 
 
@@ -1991,6 +1995,7 @@ static bool _exif_decode_exif_data(dt_image_t *img, Exiv2::ExifData &exifData)
         if(forward_suggested)
         {
           dt_control_log(_("forward matrix in `%s`, please report via darktable github"), img->filename);
+          github_request = TRUE;
         }
         dt_print(DT_DEBUG_IMAGEIO,
           "[exif] %s%s%s: selected from  [1] %s (%iK), [2] %s (%iK), [3] %s (%iK)",
@@ -2010,6 +2015,14 @@ static bool _exif_decode_exif_data(dt_image_t *img, Exiv2::ExifData &exifData)
         mat3mul(img->d65_color_matrix, illuminant_data[illuminant].CA, cameratoXYZ);
         _print_matrix_data("dt_image_t d65_color_matrix", 0, img->d65_color_matrix);
       }
+      if(github_request)
+      {
+        guint tagid = 0;
+        char tagname[32];
+        snprintf(tagname, sizeof(tagname), "darktable|github|dng_issue");
+        dt_tag_new(tagname, &tagid);
+        dt_tag_attach(tagid, img->id, FALSE, FALSE);
+      }
     }
 
     gboolean is_monochrome = FALSE;