The server's idea of the cursor position is never really initialized. Normally that's not a problem, since the first mouse movement and a number of other X events will update it. But in the case where the user never moves the mouse, it can remain out of sync. That manifests as `GetCursorPos` bouncing between the server's incorrect value and the real one from the user driver, depending on the age of the position on the server.
I've marked this as a draft for two reasons: 1. I don't quite know where this belongs. We want to do it early in the lifespan of a user driver but after there's a desktop, so I've put it behind a `pthread_once` called from `get_desktop_window`. 2. I'm calling `NtUserSetCursorPos` to update the server, rather than a direct `set_cursor` request, because it contains some DPI logic. That has the side effect of potentially warping the cursor. That's not desirable or necessary, but it shouldn't be a problem. It's easily factored around if we want to avoid it.
Any guidance would be greatly appreciated!
-- v2: win32u: Initialize the server's cursor position from the user driver's.