input: rework mouse button/motion handling

Store a list of currently pressed buttons, and which surface they
belong to (i.e. which surface that received the press).

Then, in motion events (with a button pressed, aka drag operations),
send the event to the “original” surface (that received the press).

Also send release events to the originating surface.

This means a surface receiving a press will always receive a
corresponding release. And no one will receive a release event without
a corresponding press event.

Motion events with a button pressed will *always* use the *first*
button that was pressed. I.e. if you press a button, start dragging,
and then press another button, we keep generating motion events for
the *first* button.

wayland.c

@@ -138,6 +138,8 @@ seat_destroy(struct seat *seat)
     if (seat == NULL)
         return;
 
+    tll_free(seat->mouse.buttons);
+
     tll_foreach(seat->kbd.bindings.key, it)
         tll_free(it->item.bind.key_codes);
     tll_free(seat->kbd.bindings.key);