pangowin32-fontmap.c: Allow some support for loading custom fonts

Hi,

From the commit message:

Previously, when using GDI, we are able to add custom fonts to the system font collection using AddFontResource()/AddFontResourceEx(), but this is not enough as we transition to DirectWrite

Allow some support for this for people that are using AddFontResource() (but not yet AddFontResourceEx()), by using the GDI interop support in DirectWrite.

Let people know that currently AddFontResource() should be used for the moment for this support to work, not AddFontResourceEx().

Partially fixes issue #720 720.