touch: treat a compositor re-grab down as a drag continuation

When the surface that owns a touch drag is destroyed mid-gesture, the compositor re-opens the touch grab by issuing a fresh `down` on the surface now under the finger (smithay pins touch focus at `down` and won't re-target on motion). That `down` arrives for a slot the main surface already holds as its primary, with the drag state already migrated here. Handling it through the normal `down` path would either restart the gesture or, since the slot is taken, demote it to an auxiliary touch — both abandon the in-flight drag.
`TouchHandler::down` now early-returns when the focused surface's `primary_touch_id` already equals the incoming slot, so the redundant `down` is a no-op and the following motion/up keep driving the migrated drag. Completes the cross-surface drag fix whose other half (migrating `primary_touch_id` on overlay teardown) already landed.

src/input/touch/mod.rs

@@ -58,6 +58,14 @@ impl<A: App> TouchHandler for AppData<A>
 		let pos = self.surface( focus ).to_physical( position.0, position.1 );
 		self.pointer_pos = pos;
 
+		// Compositor re-grab after the origin surface died mid-drag: this
+		// slot is already our primary and the drag state was migrated here,
+		// so treat the redundant `down` as a continuation, not a restart.
+		if self.surface( focus ).primary_touch_id == Some( id )
+		{
+			return;
+		}
+
 		// Auxiliary slot path: a second (or third…) finger arriving
 		// while another finger already drives the primary slot stays
 		// out of the single-slot gesture machine entirely. The app