[gtk3] gdkdevice-wayland: Update pointer scale on tablet tool proximity events

When the user approaches a tablet tool to the screen we get a proximity-in event and in this moment we need to check the surface output scale to find the scaling to be applied to the cursor. And the same should be done when the tool is detached or the monitors configuration changes.

#1675 (closed)

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Gtk backport of !582 (merged)