This improves implicit node inputs in multiple ways:
* Fix crash when switching a group input socket type from e.g. vector to
integer, while the default input is set to "position". Now, the default input
type is reset automatically if it's invalid.
* Add Left/Right Handle as possible implicit vector inputs (next to Position and
Normal). Those were the only ones that we used internally that were not
exposed yet.
* When creating a new group input from an existing socket, also initialize the
default input based on the socket. E.g. when grouping a `Set Position` node,
the `Position` input of the group will now also use the position attribute by
default.
In addition to these user-level changes, some internal changes were done too:
* Use unified `NodeDefaultInputType` in node declaration instead of function
pointers which were hard to propagate to node groups.
* Use a new reusable `socket_type_supports_default_input_type` function in rna
to filter the list of possible input items.
Pull Request: https://projects.blender.org/blender/blender/pulls/139139
c55ffb5258