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
8e2c9f2dd3