[server] Send pointer events to all wl_pointer resources of a client

Summary:
This is what Weston does. With this change clients can create
multiple wl_pointer instances and thus get events reported to all of them.
This will be needed to e.g. support drag on empty area in Breeze on Wayland.

A similar change was done for wl_keyboard already in 862bb68e.

Test Plan: Seat auto test still passes

Reviewers: Plasma on Wayland

Subscribers: plasma-devel

Tags: Plasma on Wayland

Differential Revision: https://phabricator.kde.org/D3016