[HarfBuzz] Caching of glyphmetrics and cmap data

[Keith Stribley]

Hi Behdad,

I've been comparing the rendering times taken by Harfbuzz-NG with 
different fonts and documents and as part of this experimented with 
caching for the freetype face and font. A lot of the time seems to be 
taken by calls to FreeType at the moment, specifically FT_Get_Char_Index 
and FT_Load_Glyph.

I found that by caching both glyphmetric lookups (in the font) and the 
unicode to glyph mapping (in the face) I was able to improve the 
rendering performance by between 8 and 50 times depending on the font in 
large documents which don't require a lot of OT lookups.

I've attached the patch that I was using. Perhaps it might be worth 
adding something like this as an option for clients for which memory is 
not at a high premium. The patch will probably need extending to benefit 
CJK and vertical metric fonts.

Regards,
Keith


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