Fixes#965
When processing keybindings that closed the surface (`close_surface`,
`close_window`), the surface and associated runtime structures would be
freed so we could segfault.
This PR introduces a new enum result for input events (only key for now)
that returns whether an event resulted in a close. In this case, callers
can properly return immediately and avoid writing to deallocated memory.
9de5d991a2