STL: correctly align at the bottom

Close #9833

modules/codec/stl.c

@@ -164,6 +164,7 @@ static subpicture_t *Decode(decoder_t *dec, block_t **block)
         sub->p_region->psz_text = ParseText(payload,
                                             payload_size,
                                             cct_nums[dec->p_sys->cct - CCT_BEGIN].str);
+        sub->p_region->i_align = SUBPICTURE_ALIGN_BOTTOM;
         sub->p_region->psz_html = NULL;
     }