Only disable drop shadow on macOS when window background opacity is transparent

[fanzeyi]

as title.

see #310

Screen.Recording.2021-01-29.at.7.57.12.PM.mov

[wez]

I'm OK with the spirit of this change but I'm not sure about the implementation.
There are two main things about it that don't sit well with me:

• The added method to control the opacity doesn't change the opacity at all (that's a function of the alpha in the renderer)
• There's some fan-out through to all of the different platforms from the change in the construction of the window

What do you think about exposing the opacity configuration through the relatively recently introduced WindowConfiguration trait for the window layer over here:

https://github.com/wez/wezterm/blob/master/window/src/configuration.rs#L20

That way we don't need to add more parameters to the constructor.
I think the problem then becomes with letting the window layer know that the config changed.

So, perhaps a method like config_did_change or advise_or_config_change or some better name could be added to WindowOps and WindowOpsMut would do the job?

In the macos implementation of that method you'd use the WindowConfiguration trait to see what the opacity is set to and then window.setHasShadow_ and window.setOpaque_ accordingly.

[wez]

Thanks!