windows.screens: Add word to get dpi on Windows.

basis/windows/screens/authors.txt

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+Doug Coleman

basis/windows/screens/platforms.txt

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+windows

basis/windows/screens/screens.factor

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+! Copyright (C) 2018 Doug Coleman.
+! See http://factorcode.org/license.txt for BSD license.
+USING: alien.c-types alien.data arrays classes.struct kernel
+windows.errors windows.shcore windows.types windows.user32 ;
+IN: windows.screens
+
+: get-dpi ( -- dpi )
+    0 0 POINT <struct-boa> 0 MonitorFromPoint
+    MDT_EFFECTIVE_DPI 
+    0 uint <ref> 0 uint <ref> [ GetDpiForMonitor win32-error=0/f ] 2keep
+    [ uint deref ] bi@ 2array ;