How to get position of tablet pen relative to surfaces?

[ferreiradaselva]

I made a libinput context that I use to get the coordinates of the (wacom)
tablet pen with `libinput_event_tablet_tool_get_x()` and
`libinput_event_tablet_tool_get_y()`.

Since libinput knows nothing of surfaces, the value is obviously the global
position of the pen. If I had the global position of the surface, I could
subtract the position of the pen relative to the surface, but according with the
docs:

"...Clients don't know the global position of their surfaces..."

> https://wayland.freedesktop.org/docs/html/ch04.html#sect-Protocol-Surface

Is there any way to solve this problem? I'm currently using `wl_surface` with
`xdg_surface` and `xdg_toplevel` roles.

Thanks in advance,

- Felipe
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