This changes is needed to give more control to modifiers' writing
callback when defined. It will allow to implement better culling of
needless data when writing e.g. modifiers from library overrides.

Ref. T97967.

Note: The actual change of behavior in MeshDeform modifier will be
comitted separately. Other modifiers will need similar changes too.
It is only included in this review as an example of why this refactor
is needed.