Fix fallback positioning of double diacritic marks

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=787284

src/hb-ot-shape-fallback.cc

@@ -218,10 +218,10 @@ position_mark (const hb_ot_shape_plan_t *plan,
     case HB_UNICODE_COMBINING_CLASS_DOUBLE_BELOW:
     case HB_UNICODE_COMBINING_CLASS_DOUBLE_ABOVE:
       if (buffer->props.direction == HB_DIRECTION_LTR) {
-	pos.x_offset += base_extents.x_bearing - mark_extents.width / 2 - mark_extents.x_bearing;
+	pos.x_offset += base_extents.x_bearing + base_extents.width - mark_extents.width / 2 - mark_extents.x_bearing;
         break;
       } else if (buffer->props.direction == HB_DIRECTION_RTL) {
-	pos.x_offset += base_extents.x_bearing + base_extents.width - mark_extents.width / 2 - mark_extents.x_bearing;
+	pos.x_offset += base_extents.x_bearing - mark_extents.width / 2 - mark_extents.x_bearing;
         break;
       }
       HB_FALLTHROUGH;

test/shaping/tests/fallback-positioning.tests

 fonts/sha1sum/8228d035fcd65d62ec9728fb34f42c63be93a5d3.ttf::U+0078,U+0301,U+0058,U+0301:[x=0+1030|acutecomb=0@-21,-27+0|X=2+1295|acutecomb=2@-147,320+0]
+fonts/sha1sum/856ff9562451293cbeff6f396d4e3877c4f0a436.ttf::U+0061,U+035C,U+0062:[uni0061=0+512|uni035C=0@-64,-128+0|uni0062=2+512]