We need the font for glyph lookup during GSUB pauses in Indic shaper.
Could perhaps be avoided, but at this point, we don't mean to support
separate substitute()/position() entry points (anymore), so there is
no point in not providing the font to GSUB.

It was impossible to meaningfully use them from the outside these days.

To be moved to initial_reordering next...

Move all the logic that needs to eventually move into the indic table
into hb-ot-shape-complex-indic-private.hh.

Comes with some 10% speedup for Devanagari even!

Gives me a good 10% speedup for the Devanagari test case.  Less so
for less lookup-intensive tests.

For the Devanagari test case, the false positive rate of the GSUB digest
is 4%.

Implement two set digests, and one that combines the two.

Fallback mark positioning works now...  With hb-ft and hb-view /
hb-shape at least.

If there is no GPOS, zero mark advances.

If there *is* GPOS and the shaper requests so, zero mark advances for
attached marks.

Fixes regression with Tibetan, where the font has GPOS, and marks a
glyph as mark where it shouldn't get zero advance.

Enabled for all shapers except for Indic.

Still working on it.

Based on patch from Philip Withnall.
https://bugs.freedesktop.org/show_bug.cgi?id=41095

Add a shaper class struct.