Clients crash when switching tty's on sway

To reproduce:

1. Start sway
2. Open a Qt application
3. Move the mouse over it
4. Switch to a tty
5. Switch back to sway
6. Click the mouse

Causes a crash

This happens because sway doesn't send enter event when switching back from a tty, leading to a nullptr dereference of the pointer focus surface. This is arguably sway misbehaving, but our clients should not crash. An stderr warning about broken compositor behavior would probably be best.