This adds support for Unicode 8.0 and fixes a bug with wrong properties
for ranges of codepoints.

Ouch!

New Tai Lue changed encoding to visual, boring, model.

Ouch.

Fixes https://github.com/behdad/harfbuzz/issues/63

We can't delete things before hb_ot_layout_position_finish().  So,
just zero the advance before it, and remove later.

Was broken by 82b521ae, as we have
positioning data by then and can't use the output buffer.  Ouch!

For example, fixes the following sequence with Arial XP:
628 25cc 651 25cc 64e 3a 20 628 651 34f 64e
628 25cc 64e 25cc 651 3a 20 628 64e 34f 651

Discovered as part of:
https://bugs.freedesktop.org/show_bug.cgi?id=85873

TODO: Documentation.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=71445

Based on patch from Martin Hosken, with review from Jonathan and I.

This works per-syllable as per the spec, but we think it should be per
spacing/base/??? glyph instead.

Use it in USE.

Same logic as in Indic shaper.

Same logic as Indic and SEA.

These were never tested with Indic shaper, and indeed wouldn't work there
because they didn't have their viramas and other config defined.  They are
all also supported by MS through USE, so route them there.