win32: Fix synaptics trackpad issues

win32: Fix synaptics trackpad issues

The synaptics trackpad driver has some weird behaviour on scroll.
It pops up a window over the mouse pointer (looking like a scrollbar).
This has two problems:
* We get extra enter/leave events for the trackpad window
* We get back the trackpad window when we look for the window
  under the mouse to deliver the mousewheel message.

So, we add some trackpad specific hacks to avoid this (sigh) based
on the trackpad window window class.

This fixes bug #542777 and was partially based on a patch there
from Peter Clifton.