windowsrawinput: Unregister/register raw input devices as necessary.

The thread is still created once and only shut down on WIN_VideoQuit,
but the thread will unregister the devices when they're disabled to avoid
wasting time processing raw input that won't be appreciated by anyone.

This also allows RIDEV_NOLEGACY to be set on the devices, otherwise they
swallow up the WM_MOUSE* events that would be going to the main thread
when relative mode is disabled. That's out of scope for this change, though.

Author: whrvt