Quick fix for issue #14 An error occurred

Page DPI is now saved as the DPI that was read when opening the file.
While not a perfectly fool-proof fix, it will at least allow files to be
migrated between OB 1.02 and 1.10.

src/adaptors/UBSvgSubsetAdaptor.cpp

@@ -1034,7 +1034,11 @@ void UBSvgSubsetAdaptor::UBSvgSubsetWriter::writeSvgElement()
     mXmlWriter.writeAttribute(UBSettings::uniboardDocumentNamespaceUri, "crossed-background", mScene->isCrossedBackground() ? xmlTrue : xmlFalse);
 
     QDesktopWidget* desktop = UBApplication::desktop();
-    mXmlWriter.writeAttribute("pageDpi", QString("%1").arg((desktop->physicalDpiX() + desktop->physicalDpiY()) / 2));
+
+    if (UBSettings::settings()->pageDpi->get() != 0)
+        mXmlWriter.writeAttribute("pageDpi", (UBSettings::settings()->pageDpi->get()).toString());
+    else
+        mXmlWriter.writeAttribute("pageDpi", QString("%1").arg((desktop->physicalDpiX() + desktop->physicalDpiY()) / 2));
 
     mXmlWriter.writeStartElement("rect");
     mXmlWriter.writeAttribute("fill", mScene->isDarkBackground() ? "black" : "white");