If the attribute already existed, but had a different domain or data type
than the user tried to write to it with (i.e. writing to `position` with
a float), then the data from field evaluation wasn't freed.
This restructures the geometry nodes modifier attribute writing process
to solve that problem and remove some of the nested if statements
that made the process confusing.
One case that still doesn't work is writing to a builtin attribute from
a different domain. If `OutputAttribute` gets that feature then that
could be supported here too.
Differential Revision: https://developer.blender.org/D14706
65e4c91bec