add support for headless windows

[lcnr]

The current tests use a hidden window.

This is not optimal as window creation fails when using github actions.

It would be ideal to use ContextBuilder::build_headless for testing.

As build_headless has a different return type than build_windowed this requires either a generic parameter on both backend::Backend and Context or a lot of conditional compilation.

[lcnr]

github actions seem to not support wayland at all.

This means that this change is not that useful and makes the API worse I am not going to
implement this for now