Fixed kerning advance in fonts with both GPOS and kern tables.

Notably, SegoeUI on Windows has kerning advances in the kern tables, but
also has a GPOS table. The pair of characters aren't the same in each
table.

stb_truetype.h

@@ -2609,14 +2609,15 @@ static stbtt_int32 stbtt__GetGlyphGPOSInfoAdvance(const stbtt_fontinfo *info, in
 
 STBTT_DEF int stbtt_GetGlyphKernAdvance(const stbtt_fontinfo *info, int g1, int g2)
 {
- int xAdvance = 0;
+ if (info->gpos) {
+ int xAdvance = stbtt__GetGlyphGPOSInfoAdvance(info, g1, g2);
+ if (xAdvance) return xAdvance;
+ }
 
- if (info->gpos)
- xAdvance += stbtt__GetGlyphGPOSInfoAdvance(info, g1, g2);
- else if (info->kern)
- xAdvance += stbtt__GetGlyphKernInfoAdvance(info, g1, g2);
+ if (info->kern)
+ return stbtt__GetGlyphKernInfoAdvance(info, g1, g2);
 
- return xAdvance;
+ return 0;
 }
 
 STBTT_DEF int stbtt_GetCodepointKernAdvance(const stbtt_fontinfo *info, int ch1, int ch2)