When calling stbtt_PackFontRanges, multiple missing glyphs in the range
of codepoints will create multiple copies of the font's missing glyph to
be added to the pixel buffer. Instead, the first codepoint that maps to the missing glyph will add it to the pixel buffer, and all subsequent glyphs will simply copy the stbtt_packedchar data to reference the same region of the buffer.
This does NOT prevent duplication in multiple calls to stbtt_PackFontRange(s) - that would require modifying the packing context, which could be nice but is a bit more intrusive.