Support fonts with more than 64k glyphs

Open Behdad Esfahbod requested to merge behdad1/freetype:beyond64k into master

Opening this also to solicit feedback.

This implements the following three extensions that HarfBuzz implements:

With these, FreeType is able to render fonts with more than 64k glyphs, the way HarfBuzz is able to.

Still need to sort out hmtx entries without lsb as well as extending last entry of hmtx for all glyphs.

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