Don't attempt to cache glyph metrics for FONT_INVALID_CODE
This was causing massive slowdown in redisplay when eg #xfe0f (VARIATION SELECTOR-16) was present, as the cache ended up very large, unused, and being recreated on every call to font_fill_lglyph_metrics (Bug#39133). * src/composite.c (fill_gstring_body): Hoist FONT_OBJECT_P check out of loop. Calculate glyph code and check for FONT_INVALID_CODE before calling font_fill_lglyph_metrics. Pass glyph code to it. * src/font.c (font_fill_lglyph_metrics): Add code parameter, move glyph code calculation up the call stack into fill_gstring_body. * src/font.h: Adjust font_fill_lglyph_metrics prototype.
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