Requested by Behdad.
Modern bitmap-only SFNTs like `NotoColorEmoji.ttf' don't contain
entries in the bitmap strike(s) for empty glyphs. Instead, they
rely that a space glyph gets created from the font's metrics data.
This commit makes FreeType behave accordingly.
* include/freetype/fterrdef.h (FT_Err_Missing_Bitmap): New error
code.
* src/sfnt/ttsbit.c (tt_sbit_decoder_load_image): Change error codes
to make a distinction between a missing bitmap in a composite and a
simple missing bitmap.
* src/truetype/ttgload.c (TT_Load_Glyph): For a missing bitmap (in a
bitmap-only font), synthesize an empty bitmap glyph if metrics are
available.
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