screenshot: Get cursor texture from window

When a window is in the background and should not have the cursor on top
of it, its _cursor will be null. By getting the texture through it, we
add this extra check, which was missing before, leading to a cursor
drawn at 0, 0 on windows where it should not have been drawn.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2702>
(cherry picked from commit 237c3e95)