On Windows, calls to `createSystemTypefaceFor` with a data buffer always
create a WindowsTypeface instance. However, calls with an existing
`Font` instance may try to create a WindowsDirectWriteTypeface, and will
only fall back to using a WindowsTypeface on failure.

Previously, a missing typeface wasn't treated as a failure, which meant
that `WindowsDirectWriteTypeface` would fall back to the first usable
typeface it could find.

With this change applied, we check whether the
WindowsDirectWriteTypeface actually managed to find the font we
requested, and will fall back to using a plain WindowsTypeface in that
case.

This patch should resolve an issue where the popupmenu for a combobox
could become 'stranded' if the plugin window was moved while the box was
open.