Attributes: Add function to rename attribute

This is implemented by removing the attribute and adding it again with
a different name. In the expected case though, implicit sharing is used
to avoid copying the array.

For now this doesn't rename UV sublayers or replace active/default color
attribute names. It's not clear where that should happen, but for now
things are clearer if those stay at a higher level.

source/blender/blenkernel/BKE_attribute.hh

@@ -653,6 +653,11 @@ class MutableAttributeAccessor : public AttributeAccessor {
     return {};
   }
 
+  /**
+   * Replace the existing attribute with a new one with a different name.
+   */
+  bool rename(const AttributeIDRef &old_attribute_id, const AttributeIDRef &new_attribute_id);
+
   /**
    * Create a new attribute.
    * \return True, when a new attribute has been created. False, when it's not possible to create

source/blender/blenkernel/intern/attribute_access.cc

@@ -911,6 +911,41 @@ GSpanAttributeWriter MutableAttributeAccessor::lookup_or_add_for_write_only_span
   return {};
 }
 
+bool MutableAttributeAccessor::rename(const AttributeIDRef &old_attribute_id,
+                                      const AttributeIDRef &new_attribute_id)
+{
+  if (old_attribute_id == new_attribute_id) {
+    return true;
+  }
+  if (this->contains(new_attribute_id)) {
+    return false;
+  }
+  const GAttributeReader old_attribute = this->lookup(old_attribute_id);
+  if (!old_attribute) {
+    return false;
+  }
+  const eCustomDataType type = cpp_type_to_custom_data_type(old_attribute.varray.type());
+  if (old_attribute.sharing_info != nullptr && old_attribute.varray.is_span()) {
+    if (!this->add(new_attribute_id,
+                   old_attribute.domain,
+                   type,
+                   AttributeInitShared{old_attribute.varray.get_internal_span().data(),
+                                       *old_attribute.sharing_info})) {
+      return false;
+    }
+  }
+  else {
+    if (!this->add(new_attribute_id,
+                   old_attribute.domain,
+                   type,
+                   AttributeInitVArray{old_attribute.varray})) {
+      return false;
+    }
+  }
+  this->remove(old_attribute_id);
+  return true;
+}
+
 fn::GField AttributeValidator::validate_field_if_necessary(const fn::GField &field) const
 {
   if (function) {