add getattributebyindex and fix afterallocate

[YassineMankai]

This repository is only used as a mirror of git.blender.org. Blender development happens on https://developer.blender.org.

To get started with contributing code, please see: https://wiki.blender.org/wiki/Process/Contributing_Code

[eliemichel]

/!\ Breaking change, requires to update plugins