This is a slightly different issue to ours, but I definitely see the need for this also.. and we could use this also.
Surely a Dropdown selector for:
"Global stock" - decrement upon order, all Variations stock qty to keep correct inventory, as set in the default variation only.
"Supplier stock" - decrement just the Variations stock qty for the Supplier for each order, as set under Supplier reference.
Given our need for, "Stock per Supplier reference" there needs to be another ""Stock per Supplier" checkbox for each Variation Qty as per:
Might look like this: 
This would give Prestashop the best Dropship/inventory management of any eCommerce software I've seen!