Add option to render missing glyphs with FreeSerif

[eugmes]

Originally from: https://sourceforge.net/p/fntsample/bugs/6/

Please, add an option to render missing glyphs with FreeSerif (or another font for which the complete TTF or OTF font path is specified ) on a red background. Implementing this will inform the using what the missing glyphs should look like or represent.

Note that the red background should differ when printed in gray scale from the currently used blue, gray and black backgrounds. For example the color #FF8888 could be used for this.