Whether to exclude invalid glyphs from our lookup tables.
When fonts are subsetted the glyphs in the tables may be
rendered invalid because they are not used. The encoding continues
to exist but the information required to draw the glyphs has been
lost. As such it is not a good idea to attempt to use these glyphs
even though they may appear to be part of the font.
By default we attempt to detect such glyphs and exclude them
from any operations or font properties such as the EncodingToChar and CharToEncoding mappings. This prevents
them being used inadvertently.