Don't use Windows-specific calls if not on Windows

- don't use Win32-specific interop calls to determine the DPI
  if not on Windows
- possible fix for #427
- add a barebones unit test for this
- note that even the Windows code isn't correct as soon as you have multiple screens
- on macOS, causes 20 instead of 5 unit tests to pass

Source/SvgDocument.cs

@@ -29,12 +29,29 @@ public class SvgDocument : SvgFragment, ITypeDescriptorContext
         private Dictionary<string, IEnumerable<SvgFontFace>> _fontDefns = null;
 
         private static int GetSystemDpi()
-        {
-            IntPtr hDC = GetDC(IntPtr.Zero);
-            const int LOGPIXELSY = 90;
-            int result = GetDeviceCaps(hDC, LOGPIXELSY);
-            ReleaseDC(IntPtr.Zero, hDC);
-            return result;
+        {
+            bool isWindows;
+
+#if NETCORE
+            isWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
+#else
+            isWindows = true;
+#endif
+
+            if (isWindows)
+            {
+                // NOTE: starting with Windows 8.1, the DPI is no longer system-wide but screen-specific
+                IntPtr hDC = GetDC(IntPtr.Zero);
+                const int LOGPIXELSY = 90;
+                int result = GetDeviceCaps(hDC, LOGPIXELSY);
+                ReleaseDC(IntPtr.Zero, hDC);
+                return result;
+            }
+            else
+            {
+                // hack for macOS and Linux
+                return 96;
+            }
         }
 
         [DllImport("gdi32.dll")]
@@ -104,7 +121,7 @@ public void OverwriteIdManager(SvgElementIdManager manager)
         /// </summary>
         public string ExternalCSSHref { get; set; }        
 
-        #region ITypeDescriptorContext Members
+#region ITypeDescriptorContext Members
 
         IContainer ITypeDescriptorContext.Container
         {
@@ -136,7 +153,7 @@ object IServiceProvider.GetService(Type serviceType)
             throw new NotImplementedException();
         }
 
-        #endregion
+#endregion
 
         /// <summary>
         /// Retrieves the <see cref="SvgElement"/> with the specified ID.

Tests/Svg.UnitTests/DpiTest.cs

@@ -0,0 +1,21 @@
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+namespace Svg.UnitTests
+{
+    /// <summary>
+    /// Test that basic graphics operations work. Currently only supported
+    /// on Windows, macOS, and Linux.
+    /// </summary>
+    [TestClass]
+    public class DpiTest
+    {
+        /// <summary>
+        /// We should get a valid dpi (probably 72, 96 or similar).
+        /// </summary>
+        [TestMethod]
+        public void TestDpiAboveZero()
+        {
+            Assert.IsTrue(SvgDocument.PointsPerInch > 0);
+        }
+    }
+}