Support relative mouse mode on Windows

[rdb]

Windows is the only desktop platform we don't support relative mouse mode on. This is a constant stumbling block for anyone who is making a game with FPS-style input.

It's probably pretty easy to add this by just enabling cursor confinement and using WM_INPUT events. This would be similar how we handle it in the X11 implementation nowadays when DGA is not supported.

[ekiefl]

Thanks for addressing this. Is the commit 3000aa2 present in any release?

[rdb]

No, it will be part of Panda3D 1.11.