fix: provide default display handle for web canvas surface targets

[robertpakalns]

Description
Since wgpu version 29, creating a surface from a web canvas using Instance::create_surface with SurfaceTarget::Canvas or SurfaceTarget::OffscreenCanvas fails with CreateSurfaceError::MismatchingDisplayHandle.

This occurs because both SurfaceTarget::Canvas and SurfaceTarget::OffscreenCanvas set raw_display_handle: None. However, in wgpu_core::instance::Instance::create_surface, there is a requirement that either instance_display_handle or display_handle must be present. Since raw_display_handle is always None, it always matches the (None, None) => return Err(CreateSurfaceError::MissingDisplayHandle) arm.

Note: this change does not affect the examples (examples/features) because it utilizes winit which provides a valid RawDisplayHandle when creating windows (including web canvases) and which never hits the (None, None) case.

Testing
Tested in browser using:

 let surface = instance
        .create_surface(SurfaceTarget::Canvas(canvas.clone()))
        .unwrap();

Squash or Rebase?
Squash