GdkWin32: Use a signed integral type for the DPI scale

An arithmetic operation involving a signed and an unsigned operand
of the same rank will have both operands converted to *unsigned*.
That's an issue if the signed operand actually has a negative value.

That was causing issues with the handling of monitor geometries that
had negative x / y positions.

gdk/win32/gdkmonitor-win32.c

@@ -618,7 +618,7 @@ enum_monitor (HMONITOR hmonitor,
           GdkWin32Monitor *w32mon;
           GdkMonitor *mon;
           GdkRectangle rect;
-          guint scale;
+          int scale;
 
           memset (&dd_monitor, 0, sizeof (dd_monitor));
           dd_monitor.cb = sizeof (dd_monitor);