Add placeholder black color for picture thumbnails (for now)

src/photo.c

@@ -18,6 +18,8 @@ void cammy_photo_export(cammy_photo *src,
 }
 
 void cammy_photo_import(cammy_photo *dest, uint8_t *src, int depth) {
+    memset(&dest->thumb, '\xff', sizeof(dest->thumb));
+
     cammy_image_dither_to_tile((cammy_tile *)&dest->tiles,
                                src,
                                CAMMY_PHOTO_WIDTH,