Blender Git Loki

Fix T90907: RNA pointers for material slots are not unique across IDs

Caused by {rB1a81d268a19f}.

This caused e.g. ALT-clicking the 'Link' button to not propagate to
other selected objects (same as the 'Copy To Selected' context menu
entry).

If these are not unique across IDs, checks in ui_selectcontext_begin()
or copy_to_selected_button() could fail.

Now offset by ID pointer.

thx @JacquesLucke for clarification on the POINTER_* macros (and why
not to use them)!

Maniphest Tasks: T90907

Differential Revision: https://developer.blender.org/D12321