Added GetDeviceCaps (windows platform)

We will use this method to read out the logical DPI of the monitor and
scale window elements accordingly.

Source/OpenTK/Platform/Windows/API.cs

@@ -977,6 +977,13 @@ namespace OpenTK.Platform.Windows
 
         #endregion
 
+        #region GetDeviceCaps
+
+        [DllImport("gdi32.dll", CharSet = CharSet.Auto, SetLastError = true, ExactSpelling = true)]
+        public static extern int GetDeviceCaps(IntPtr hDC, DeviceCaps nIndex);
+
+        #endregion
+
         #endregion
 
         #region Input functions
@@ -2971,6 +2978,16 @@ namespace OpenTK.Platform.Windows
 
     #endregion
 
+    #region DeviceCaps
+
+    enum DeviceCaps
+    {
+        LogPixelsX = 88,
+        LogPixelsY = 90
+    }
+
+    #endregion
+
     #region internal enum DisplayModeSettingsEnum
 
     internal enum DisplayModeSettingsEnum