When setting these materials, then removing the modifier, usercount
would be decreased, resulting in possible dataloss on reload
Affected were:
- all Grease Pencil modifiers material influence materials
- `LineartModifier` `target_material`
- `OutlineModifier` `outline_material`
These were all using `IDWALK_CB_USER` which ends up decrementing
usercount in `modifier_free_data_id_us_cb` when the modifier is removed
So to resolve, decrement/increment material usercount in
`rna_GreasePencilModifier_material_set` appropriately
NOTE: previously, it was also doing `id_lib_extern` on the object?!
(should be on the material, no?)
NOTE: still not 100% sure where we actually use refcounting (esp. in
modifiers) and where we dont, another alternative is to just drop it and
use IDWALK_CB_NOP` (instead of `IDWALK_CB_USER`) for these materials.
Pull Request: https://projects.blender.org/blender/blender/pulls/135729
1f780a5caa