demo panics on wayland

[0x777]

Running demo/native (latest commit on master) on Arch Linux with Wayland (sway) panics with the following backtrace:

> cd demo/native
> RUST_BACKTRACE=1 cargo run --release
warning: profiles for the non root package will be ignored, specify profiles at the workspace root:
package:   /scratch/pathfinder/examples/canvas_webgl_minimal/Cargo.toml
workspace: /scratch/pathfinder/Cargo.toml
warning: profiles for the non root package will be ignored, specify profiles at the workspace root:
package:   /scratch/pathfinder/web_canvas/Cargo.toml
workspace: /scratch/pathfinder/Cargo.toml
warning: unused import: `std::mem`
  --> demo/native/src/main.rs:29:5
   |
29 | use std::mem;
   |     ^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: 1 warning emitted

    Finished release [optimized] target(s) in 0.09s
     Running `/scratch/pathfinder/target/release/demo`
The application panicked (crashed).
Message:  called `Option::unwrap()` on a `None` value
Location: /.cargo/registry/src/github.com-1ecc6299db9ec823/winit-0.19.3/src/platform/linux/wayland/window.rs:289

Run with RUST_BACKTRACE=full to include source snippets.
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ BACKTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                          (5 post panic frames hidden)                          
 5: core::panicking::panic::h7ece43057e5422d4
    at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libcore/panicking.rs:50
 6: winit::platform::platform::wayland::window::Window::get_current_monitor::h2202aa7290d19840
    at <unknown source file>
 7: winit::window::<impl winit::Window>::get_current_monitor::hbdce7b7188e31715
    at <unknown source file>
 8: demo::WindowImpl::size::heb6d19310f9f4c6f
    at <unknown source file>
 9: demo::main::h0660a1ffdb0bee78
    at <unknown source file>
10: std::rt::lang_start::{{closure}}::h612743cda8189c26
    at <unknown source file>
11: std::rt::lang_start_internal::{{closure}}::h5d3ea623498f5f43
    at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/rt.rs:52
12: std::panicking::try::do_call::hac65e71be769a440
    at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/panicking.rs:348
13: std::panicking::try::hd4706e264bcf6712
    at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/panicking.rs:325
14: std::panic::catch_unwind::h948a0fb4a8b3ee82
    at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/panic.rs:394
15: std::rt::lang_start_internal::h72cc068ed2d0ac53
    at /rustc/04488afe34512aa4c33566eb16d8c912a3ae04f9/src/libstd/rt.rs:51
16: main
    at <unknown source file>
17: __libc_start_main
    at <unknown source file>
18: _start
    at <unknown source file>

Related maybe: rust-windowing/winit#793