Inherit XeTeXFontInst_Mac from XeTeXFontInst_FT2

So that we can share the FreeType code especially for HarfBuzz, right
now on Mac HarfBuzz uses FreeType when fonts are loaded by file name and
our Core Text-based font functions when fonts are loaded by font name,
which is not good.
Remove now unused HarfBuzz font functions
Use XeTeXFontInst_FT2 for those
Revert "Implement variation selectors support"

This reverts commit 835fce61e79983a314e0866c01fb6ad502c54ff2
Make few functions static
Merge XeTeXFontInst_FT2.cpp into XeTeXFontInst.cpp
Get ‘post’ table from FreeType
Unused typedef's
Drop font table caching

There is no much point in it now.
Get more tables from FreeType
Drop no longer used header
Unused #includes