plug-ins: Match RGB FITS orientation to SiriL

Switching to the cfitsio library's fits_read_img () function in 03ea666f 
caused RGB FITS images to be read in upside down per SiriL developers and
NASA standards (odd, since cfitsio is maintained by NASA). This flips them back.

plug-ins/file-fits/fits.c

@@ -605,6 +605,17 @@ load_image (GFile        *file,
         show_fits_errors (status);
 
       g_object_unref (buffer);
+
+      if (hdu.naxisn[2] && hdu.naxisn[2] == 3)
+        {
+          /* Per SiriL developers, FITS images should be loaded from the
+           * bottom up. fits_read_img () loads them from top down, so we
+           * should flip the layer. */
+          gimp_item_transform_flip_simple (GIMP_ITEM (layer),
+                                           GIMP_ORIENTATION_VERTICAL,
+                                           TRUE, -1.0);
+        }
+
       if (pixels)
         g_free (pixels);