input/cursor: correctly transfer focus when using tablet pen
input/cursor: correctly transfer focus when using tablet pen

Fixes #4819.

This commit ensures that `seat_set_focus` is called to transfer focus
when a window is selected via a pen. Previously, it would race with
`node_at_coords`, and only properly transfer focus if its returned
`surface` was NULL.

1ace/sway,1ace/sway,1ace/sway