masks: don't let mask manager rows hand out stale module pointers

[da-phil]

The treeview caches a raw dt_iop_module_t* per row, but rows outlive modules: gui_update() only rebuilds the store when the shape structure hash changes, while modules are freed when an instance is deleted or the image changes. Deleting a shape then passed a dangling pointer to dt_masks_form_remove() and crashed in dt_iop_is_hidden().

• _lib_masks_get_values(): only return a module still present in dev->iop
• _tree_delete_shape(): stop clobbering the row's module with the one of the previous row
• _forms_structure_hash(): include the modules and their mask ids so a changed module set forces a rebuild instead of an in-place refresh

Speculatively fixes #22050

Disclaimer: this change was co-created with Claude.

[TurboGit]

Since mask manager will be removed in 5.8 and replaced by the new flexi masking maybe this is just not necessary.

[da-phil]

I guess it also depends if that gets into the way of the flexi masking development, like constantly producing merge conflicts. If not, I don't see why we should not make sure existing bugs are fixed until the module can be fully replaced by a new one. What do you think about that?