This is needed by the Vulkan backend during pipeline construction. There are dynamic states (VK_DYNAMIC_STATE_PRIMITIVE_TOPOLOGY_EXT) that can be set during creation. But this requires the VK_EXT_extended_dynamic_state Vulkan extension which may limit reach. Moreover, Impeller already has this information upfront and we never need to change the topology after pipeline construction. So this should make no impact to flexibility. Just move the option to the descriptor.