aperio: Properly map associated images in single-level slides

On single-level Aperio slides, we were dropping the thumbnail and label
and mislabeling the macro image as the thumbnail.  After encountering
exactly one tiled directory, any stripped image would be interpreted as
the thumbnail.  Instead, interpret only the second TIFF directory as the
thumbnail.

Reported-and-debugged-by: Kent Johnson <kent3737@gmail.com>

src/openslide-vendor-aperio.c

@@ -350,18 +350,19 @@ bool _openslide_try_aperio(openslide_t *osr, TIFF *tiff,
   TIFFSetDirectory(tiff, 0);
   i = 0;
   do {
+    tdir_t dir = TIFFCurrentDirectory(tiff);
     if (TIFFIsTiled(tiff)) {
-      levels[i++] = TIFFCurrentDirectory(tiff);
-      //g_debug("tiled directory: %d", TIFFCurrentDirectory(tiff));
+      levels[i++] = dir;
+      //g_debug("tiled directory: %d", dir);
     } else {
       // associated image
-      const char *name = (i == 1) ? "thumbnail" : NULL;
+      const char *name = (dir == 1) ? "thumbnail" : NULL;
       if (!add_associated_image(osr ? osr->associated_images : NULL,
                                 name, tiff, err)) {
 	g_prefix_error(err, "Can't read associated image: ");
 	goto FAIL;
       }
-      //g_debug("associated image: %d", TIFFCurrentDirectory(tiff));
+      //g_debug("associated image: %d", dir);
     }
 
     // check depth