gtk win32 & multiple-monitor support

Some display cards especially laptop/notebook support multiple monitors.

For Windows/win32, the left/top boundary of the virtual screen is:

If the desktop monitor(with taskbar & desktop icon)  is configured at the right of the blank montor, dpy_x_ofs can be negative.  In gtk/win32, the left/top corner of the virtual screen is always (0,0). The coordinate is shifted.

In other words, to convert a windows point (wx, wy) to gtk (gx, gy), you have to