public int get_unknown_glyphs_count ()
Counts the number unknown glyphs in this.
That is, zero if glyphs for all characters in the layout text were found, or more than zero otherwise.
This function can be used to determine if there are any fonts available to render all characters in a certain string, or when used in combination with FALLBACK, to check if a certain font supports all the characters in the string.
The number of unknown glyphs in this.