Fix x/y calculcation in hit testing on multiple monitors

As stated in the documentation:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms645618(v=vs.85).aspx

"Important  Do not use the LOWORD or HIWORD macros to extract the x- and y- coordinates of the cursor position because these macros return incorrect results on systems with multiple monitors. "
Merge pull request #3 from peters/patch-2

Fix x/y calculcation when hit testing form on multiple monitors