Fixed|Windows: -vdmap not working as expected

FS1::findAndOpenNativeFile() used NativePath::workPath() to ensure
that the supplied path argument was absolute. However at this point
the path should not be translated to use native separators because
internally the virtual directory mappings use forward slashes.

doomsday/engine/src/filesys/fs_main.cpp

@@ -273,7 +273,7 @@ struct FS1::Instance
         DENG_ASSERT(!path.isEmpty());
 
         // We must have an absolute path - prepend the CWD if necessary.
-        path = NativePath::workPath() / path;
+        path = NativePath::workPath().withSeparators('/') / path;
 
         // Translate mymode to the C-lib's fopen() mode specifiers.
         char mode[8] = "";