Attributes: Reset various runtime data for writing files

These fields are always reset at runtime. Writing zeros should be easier
to compress since a lot of other data is zeroed, and it also makes it
easier for developers to filter out noise when looking at data buffers.

Pull Request: https://projects.blender.org/blender/blender/pulls/144582

source/blender/blenkernel/intern/attribute_storage.cc

@@ -194,6 +194,7 @@ AttributeStorage::AttributeStorage()
 {
   this->dna_attributes = nullptr;
   this->dna_attributes_num = 0;
+  memset(this->_pad, 0, sizeof(this->_pad));
   this->runtime = MEM_new<AttributeStorageRuntime>(__func__);
 }
 
@@ -600,6 +601,7 @@ void attribute_storage_blend_write_prepare(AttributeStorage &data,
 
     write_data.attributes.append(attribute_dna);
   });
+  data.runtime = nullptr;
 }
 
 static void write_shared_array(BlendWriter &writer,

source/blender/blenkernel/intern/customdata.cc

@@ -5141,6 +5141,11 @@ void CustomData_blend_write_prepare(CustomData &data,
   }
   data.totlayer = layers_to_write.size();
   data.maxlayer = data.totlayer;
+  std::fill_n(data.typemap, CD_NUMTYPES, 0);
+  data.totsize = 0;
+  if (layers_to_write.is_empty()) {
+    data.layers = nullptr;
+  }
 
   /* NOTE: `data->layers` may be null, this happens when adding
    * a legacy #MPoly struct to a mesh with no other face attributes.