w32_common: add support for precise scrolling

This is pretty similar to the Wayland support added in f364a1e.

Mouse drivers with high precision scrolling are rare on Windows, but
they should report values less than WHEEL_DELTA in WM_MOUSEWHEEL. If the
mouse moves more than one notch, the driver can also send a value higher
than WHEEL_DELTA to indicate that the scroll action should be performed
more than once.