The change to use generic "capture field on geometry" utilities for this
node and other nodes like it means `AttributeWriter` with its update
tagging isn't being used anymore, the attribute is just being created
with the new values (for some cases anyway). To fix this, call the
attribute provider's update function when creating the attribute too.
This was noted as useful in 130701763b too.
The initialization of curve and point cloud runtime structs is moved
because they now have to be allocated before any attributes are added.
73cc27b988