PangoWin32: Cleanup DirectWrite code a bit

Hi,

On the way to make more use of DirectWrite in the PangoWin32 code to support more features, there are some cleanups that can be done in the process:

• We inadvertently initialized DirectWrite repeatedly, so this aims to avoid this.
• Clean up DirectWrite object initialization code (we can just store up the IDWriteFont objects that we create)
• Drop an unused variable in PangoCairoWin32.

With blessings, thank you!