Problems may occur in cases where text contains code points not
supported by any of the fonts in the logical font or a physical font + linked font.

1. If a code point that requires complex text is in the text, it, and subsequent
code points map to the unknown glyph. This is only in the hardwire pipeline
and is because the glyphCache code calls the "no shaping" mapper
which returns as soon as a code point that requires shaping is encountered.
Only code that has a way to invoke complex text rendering should do this.

2. If an EUDC font is present in the user's environment, and a code point
   is not supported by any of the other fallback resources, we try to open
   that EUDC font but fail because since GDI never enumerates an EUDC
  font we don't try opening the font. We should do so. At the same time
  it seems prudent to add checks to prevent NPEs resulting from such
  cases. These ought not to happen, and I can't think of any other case
  when it is likely but I think I'll add the check anyway.