Allow users to create viewports larger than monitor on Windows & macOS

[lopo12123]

Added clamp_size_to_monitor_size field on ViewportBuilder, which means whether clamp the window's size to monitor's size. (default to true)

• Closes Allow creation of large window that can exceed the monitor size #3389

simple example

pub struct MyApp {}

impl MyApp {
    pub fn new() -> MyApp {
        MyApp {}
    }
}

impl eframe::App for MyApp {
    fn update(&mut self, ctx: &Context, frame: &mut eframe::Frame) {
        egui::CentralPanel::default()
            .frame(Frame::none().fill(Color32::DARK_GRAY))
            .show(ctx, |ui| {
                if ctx.input(|i| i.key_pressed(Key::Escape)) {
                    ctx.send_viewport_cmd(ViewportCommand::Close);
                }
            });
    }
}

pub fn main() {
    let option = eframe::NativeOptions {
        viewport: ViewportBuilder::default()
            .with_position([10.0, 10.0])
            .with_inner_size([3000.0, 2000.0])
            .with_clamp_size_to_monitor_size(false),
        ..Default::default()
    };

    eframe::run_native(
        "a large window app",
        option,
        Box::new(|ctx| Box::new(MyApp::new())),
    ).unwrap();
}

It works on my windows (with 3 monitors), but I don't have a test environment for macos

[emilk]

This was discussed somewhere

[emilk]

It only default to true on Linux

[lopo12123]

updated

[emilk]

need cargo fmt