BLI: support smart pointer for weak implicit sharing pointer

Previously, one always had to do manual user management for weak users.

source/blender/blenlib/BLI_implicit_sharing_ptr.hh

@@ -21,7 +21,7 @@ namespace blender {
  * types that derive from #ImplicitSharingMixin. It is fairly similar to #std::shared_ptr but
  * requires the reference count to be embedded in the data.
  */
-template<typename T> class ImplicitSharingPtr {
+template<typename T, bool IsStrong = true> class ImplicitSharingPtr {
  private:
   const T *data_ = nullptr;
 
@@ -117,24 +117,51 @@ template<typename T> class ImplicitSharingPtr {
     return get_default_hash(data_);
   }
 
+  static uint64_t hash_as(const T *data)
+  {
+    return get_default_hash(data);
+  }
+
   BLI_STRUCT_EQUALITY_OPERATORS_1(ImplicitSharingPtr, data_)
 
+  friend bool operator==(const T *a, const ImplicitSharingPtr &b)
+  {
+    return a == b.data_;
+  }
+
+  friend bool operator==(const ImplicitSharingPtr &a, const T *b)
+  {
+    return a.data_ == b;
+  }
+
  private:
   static void add_user(const T *data)
   {
     if (data != nullptr) {
-      data->add_user();
+      if constexpr (IsStrong) {
+        data->add_user();
+      }
+      else {
+        data->add_weak_user();
+      }
     }
   }
 
   static void remove_user_and_delete_if_last(const T *data)
   {
     if (data != nullptr) {
-      data->remove_user_and_delete_if_last();
+      if constexpr (IsStrong) {
+        data->remove_user_and_delete_if_last();
+      }
+      else {
+        data->remove_weak_user_and_delete_if_last();
+      }
     }
   }
 };
 
+using WeakImplicitSharingPtr = ImplicitSharingPtr<ImplicitSharingInfo, false>;
+
 /**
  * Utility struct to allow used #ImplicitSharingPtr when it's necessary to type-erase the backing
  * storage for user-exposed data. For example, #blender::Vector, or #std::vector might be used to