Fix transparency on macOS

[wusyong]

Unlike other platforms, macOS adds another opaque layer on CALayer.
This makes the window opaque even if winit window is set to transparent.

Edit: needs #261 to fix CI.

[jdm]

I wonder if we should add a field to NativeWidget that makes these calls configurable? The docs at https://developer.apple.com/documentation/quartzcore/calayer/1410763-opaque suggest that this could have performance implications for the system compositor.

[wusyong]

Sure, I can look into adding a method to toggle it. But it'll probably only matter to macOS just like how glutin works.
glutin's x11/wayland/Windows transparency is more on winit's config.
I'm on vacation soon. I'll resume the work once I get back next week.

[wusyong]

Sorry for the late update. My laptop was broken after vacation and took me a few days to fix.
I added an opaque field to the NativeWidget. It's determined by NSWindow's opaque property.
So if the raw window hand or winit window sets the transparency, it will make the layer transparent as well.
Otherwise, normal usage is still the same.

[jdm]

Nice!