Guard against unsupported window mode on startup

[Susko3]

• Closes Platforms other than Windows crash when last windowed mode is borderless in latest master #6262

I'd have hoped this GameHost code would guard against unsupported window modes, but this runs before the window is created so it has no effect.

osu-framework/osu.Framework/Platform/GameHost.cs

Lines 1229 to 1236 in 79d8e44

	windowMode.BindValueChanged(mode =>
	{
	if (Window == null)
	return;
	if (!Window.SupportedWindowModes.Contains(mode.NewValue))
	windowMode.Value = Window.DefaultWindowMode;
	}, true);

[bdach]

@peppy this pull (or #6264, if it gets merged, I guess) must make it into a framework package before we do a game release.