Fixes https://github.com/behdad/harfbuzz/issues/253

Hopefully we got the logic right.

Fixes https://github.com/behdad/harfbuzz/issues/243

With javatext.ttf, the reodering medial Ra gets its advance width
zero'ed in Uniscribe implementation, and the font adds the advance
back.  Our Indic shaper does not do that, but USE does.  So, route
Javanese through USE.  That's what Microsoft does anyway.  Test:

  U+A9A5,U+A9BA

This also seems to fix the following sequence, and variations thereof:

  U+A99F,U+A9C0,U+A9A2,U+A9BF

Fixes https://github.com/behdad/harfbuzz/issues/251

These are frozen, so good time to add.

To make a release...

Apparently some clients have reference-table callbacks that copy the table.
As such, avoid loading 'glyf' table which is only needed if fallback positioning
happens.

Fixes https://github.com/behdad/harfbuzz/issues/237

Note that Uniscribe enforces a certain syllable order.  We don't.
But with this change, I get all of the tibetan contractions pass
with Microsoft Himalaya font.

From http://www.babelstone.co.uk/Tibetan/Contractions.html

Update opentype language tags

Fixes https://github.com/behdad/harfbuzz/issues/234

To make room to remember Mongolian Free Variation Selectors.
Part of fixing https://github.com/behdad/harfbuzz/issues/234

Unbreak build on Windows CE

0475ef2f broke the build by using
vsnprintf(), which is not defined on Windows CE

Improve DirectWrite backend

Ouch.  Had broken it in 9a13ed45

Fixes https://github.com/behdad/harfbuzz/issues/238
Will add test soon.