[atomic] Add hb_atomic_ptr_t<> and port all uses

Found and fixed a couple bugs.

Found a couple multithreading issues.  Marked them with "XXX-MT-bug".

src/hb-face.cc

diff --git a/src/hb-face.cc b/src/hb-face.cc
index e149275..49f29d3 100644
--- a/src/hb-face.cc
+++ b/src/hb-face.cc
@@ -78,12 +78,12 @@
   0,    /* num_glyphs */
 
   {
-#define HB_SHAPER_IMPLEMENT(shaper) HB_SHAPER_DATA_INVALID,
+#define HB_SHAPER_IMPLEMENT(shaper) HB_ATOMIC_PTR_INIT (HB_SHAPER_DATA_INVALID),
 #include "hb-shaper-list.hh"
 #undef HB_SHAPER_IMPLEMENT
   },
 
-  nullptr, /* shape_plans */
+  HB_ATOMIC_PTR_INIT (nullptr), /* shape_plans */
 };
 
 
@@ -249,7 +249,7 @@
 {
   if (!hb_object_destroy (face)) return;
 
-  for (hb_face_t::plan_node_t *node = face->shape_plans; node; )
+  for (hb_face_t::plan_node_t *node = face->shape_plans.get (); node; )
   {
     hb_face_t::plan_node_t *next = node->next;
     hb_shape_plan_destroy (node->shape_plan);