WinRT: switch to DirectWrite font engine

Currently, the basic FT engine is used, and it has the unfortunate requirement that fonts must be packaged with the application.

Using the DirectWrite engine should allow use of the native font database and not require any fonts in the application package.

One potential downside to this approach is that packaged fonts might not be possible to use with DirectWrite under WinRT. If this happens to be the case, there should be a way to fall back to the FT engine.