Improve on-screen rendering of fine detail

For some reason the Windows scaler wants (size + 1) rather than (size)
to yield good results with NearestNeighbor mode.  The improvement
for MacPaint images with fine detail is dramatic.

The PrintShop BARNS graphic in the test set looks significantly
better, though it still has a couple of thickened rows/columns,
e.g. right down the middle.

This only affects the file viewer display.  Exported images have
always looked correct.

cp2_wpf/FileViewer.xaml.cs

@@ -676,8 +676,11 @@ private class ConverterComboItem {
                 return;
             }
 
-            previewImage.Width = Math.Floor(bsrc.PixelWidth * mult);
-            previewImage.Height = Math.Floor(bsrc.PixelHeight * mult);
+            // I'm not sure why "+1" is needed here, but it significantly improves the
+            // appearance of B&W images with fine detail, e.g. MacPaint graphics.  This
+            // just affects the Windows on-screen scaling, not the export.
+            previewImage.Width = Math.Floor(bsrc.PixelWidth * mult) + 1;
+            previewImage.Height = Math.Floor(bsrc.PixelHeight * mult) + 1;
             //Debug.WriteLine("Gfx zoom " + mult + " --> " +
             //    previewImage.Width + "x" + previewImage.Height);
         }