Blender Git Loki

Geometry Nodes: Only create instance IDs when they exist

Instance IDs serve no purpose for rendering when they aren't stable from
one frame to the next, and if the index is used in the end anyway, there
is no point in storing a vector of IDs and copying it around.

This commit exposes the `id` attribute on the instances component,
makes it optional-- only generated by default with the distribute points
on faces node.

Since the string to curves node only added the index as each instance's
ID, I removed it. This means that it would be necessary to add the ID
data manually if the initial index actually helps (when deleting only
certain characters, for example).

Differential Revision: https://developer.blender.org/D12980