simple_layer example: Handle 0 width/height in configure correctly

[bbb651]

The wlr-layer-shell-unstable protocol says:

If the width or height arguments are zero, it means the client should decide its own window dimension.

While the wording doesn't make it clear, I think it's reasonable to assume it's talking about each dimension separately - the word "dimension" isn't plural, and there's legitimate use cases for this behavior (e.g. scrolling compositors like niri).

Relevent issue: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/issues/155 (about xdg-shell not layer-shell, the wording comes from there)

This can also be written as:

self.width = Some(configure.new_size.0).filter(|width| *width != 0).unwrap_or(256);
self.height = Some(configure.new_size.1).filter(|height| *height != 0).unwrap_or(256);

or plain ifs, which might be a little bit more readable, but I feel like using and encouraging NonZeroU32 is more idiomatic and flexible. It's not clear what's the MSRV policy, but I avoided NonZero since it's pretty new (rust 1.79.0).