Attributes: Add function to retrieve span without adding attribute

Previously, the only versions of attribute access that gave a span would
also add the attribute when it doesn't exist, which isn't always wanted.

source/blender/blenkernel/BKE_attribute.hh

@@ -552,6 +552,11 @@ class MutableAttributeAccessor : public AttributeAccessor {
    */
   GAttributeWriter lookup_for_write(const AttributeIDRef &attribute_id);
 
+  /**
+   * Same as above, but returns a type that makes it easier to work with the attribute as a span.
+   */
+  GSpanAttributeWriter lookup_for_write_span(const AttributeIDRef &attribute_id);
+
   /**
    * Get a writable attribute or non if it does not exist.
    * Make sure to call #finish after changes are done.
@@ -568,6 +573,19 @@ class MutableAttributeAccessor : public AttributeAccessor {
     return attribute.typed<T>();
   }
 
+  /**
+   * Same as above, but returns a type that makes it easier to work with the attribute as a span.
+   */
+  template<typename T>
+  SpanAttributeWriter<T> lookup_for_write_span(const AttributeIDRef &attribute_id)
+  {
+    AttributeWriter<T> attribute = this->lookup_for_write<T>(attribute_id);
+    if (attribute) {
+      return SpanAttributeWriter<T>{std::move(attribute), true};
+    }
+    return {};
+  }
+
   /**
    * 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

@@ -875,6 +875,16 @@ GAttributeWriter MutableAttributeAccessor::lookup_for_write(const AttributeIDRef
   return attribute;
 }
 
+GSpanAttributeWriter MutableAttributeAccessor::lookup_for_write_span(
+    const AttributeIDRef &attribute_id)
+{
+  GAttributeWriter attribute = this->lookup_for_write(attribute_id);
+  if (attribute) {
+    return GSpanAttributeWriter{std::move(attribute), true};
+  }
+  return {};
+}
+
 GAttributeWriter MutableAttributeAccessor::lookup_or_add_for_write(
     const AttributeIDRef &attribute_id,
     const eAttrDomain domain,