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.