linux/x11: Fix bugs related to unflushed commands (#13844)

**Edit**:

This PR adds flushes to functions which should have an immediate affect.
I've observed it fixing the following bugs (but there are probably
more):

- The cursor not updating when just hovering.
- The window not maximising after clicking the full-screen button until
you move the mouse.
- The window not minimising after clicking the minimise button until you
move the mouse.

---

**Original content**:

Following #13646, the cursor style wouldn't change because the
`change_window_attributes` command wasn't being flushed. I guess it was
working before because something else was flushing it somewhere else so
it was never noticed.

I just added `check()` which flushes the command so that the cursor will
actually update when just hovering. Before you would need to interact
with the window so that something else could flush the command.

Release Notes:

- N/A