Before this patch gets totally lost...

Original message said:

I changed pango_ot_ruleset_shape to use the GPOS info if available, but
there's none to use, and I don't know how to synthesize it (is there a way
to stick that sort of stuff into the ruleset?)

I've got a patch that basically has pango_ot_ruleset_shape doing basic
shaping, and then arabic-xft fixing up the positioning on the non-spacing
marks. The problem I've run into is that in order to figure out whether a
fixup is needed on a particular glyph (via g_unichar_type())  one needs to
do glyph <-> unichar mapping, which I don't know how to do 100% of the
time, since I think there are cases where num_glyphs != num_chars.

There is also the issue of whether handling these non-spacing marks could
be done generically - I don't know enough about how most languages use
these marks to say whether that is possible.

Patch thus far attached, comments requested.