[HarfBuzz] Arabic presentation forms and the uniscribe backend

[Jonathan Kew]

I'm seeing some problems with the uniscribe backend when using Arabic 
presentation-form codepoints; in many cases, the glyph positioning seems 
to be badly off.

For example, see the Uniscribe rendering at

http://ec2-54-226-13-158.compute-1.amazonaws.com/testcase-view.html?file=out/fonts/win8/arabtype.ttf/texts/arab/wikipedia/ur.txt.diffs#L_1_41824

where there's a large gap between the last two glyphs. There are many 
similar examples in the list there, too.

This problem does NOT appear when using the same text and font in 
Firefox with uniscribe shaping, which makes me suspect it's a glitch of 
some kind in the harfbuzz-uniscribe backend.