add option for undecorated_shadow on windows

[n1ght-hunter]

adds an option to enable undecorated_shadow on windows. something that is needed for client decorations. this could actually be done on the client side though. since it just makes the window smaller and adds a 1px border around the edge.

[n1ght-hunter]

also there is something about shadow on macos but its not documented properly so have no idea what it is. if someone is able to test that would be great i dont have a macos pc available.

link to the code in winit https://github.com/rust-windowing/winit/blob/a127bd6f6684909852000cdf32469bac3b611887/src/platform/macos.rs#L256

[dtzxporter]

Hi, it works similarly on macOS. The only difference is, on macOS the default is true where on win32 the default is false.