We often had to use two `FieldEvaluator` instances to first evaluate the selection and then the remaining fields. Now both can be done with a single `FieldEvaluator`. This results in less boilerplate code in many cases. Performance is not affected by this change. In a separate patch we could improve performance by reusing evaluated sub-fields that are used by the selection and the other fields. Differential Revision: https://developer.blender.org/D13571