fix permissions on files

Source files and .ttf files need not be executable
cache the glyph id and use it for kerning

When using stbtt_GetCodepointKernAvance, sbt_truetype
has to convert both codepoints internally to glyph ids
with sbtt_FindGlyphIndex, which is a rather heavyweight
function that can be entirely avoided by caching the
glyph and calling stbtt_GetGlyphKernAdvance.
Merge branch 'cache-glyph-id-and-fix-make' of https://github.com/Aktau/fontstash into Aktau-cache-glyph-id-and-fix-make