[HarfBuzz] Thai Tam Shaping

[Simon Cozens]

The Lanna for Lanna is "ᩋᩣᨱᩣᨧᩢᨠᩕᩃ᩶ᩣ᩠ᨶᨶᩣ"[*].

I'm getting very different renderings of that string in Safari, Firefox
and hb-shape, using the same fonts. I don't know which is correct.
Notice in the attached PNG that not only is HB giving me no-base-glyph
dotty circles, the MEDIAL RA (the thing that looks like a ] ) is moves
all over the place.

Harfbuzz is HEAD, with graphite2. Firefox is 41.0.2, with its own Harfbuzz.

Here's the hb-shape:

hb-shape --font-funcs=ft
/Users/simon/Library/Fonts/NotoSansTaiTham-Regular.ttf

[gid47=0+1812|gid70=0+1006|gid21=2+2236|gid70=2+1006|gid11=4+1220|gid69=4 at -140,0+0|gid4=6 at 440,0+1896|gid57=6 at -1920,0+484|gid39=8+1839|gid89=8 at -559,0+0|gid135=8+1217|gid70=8+1006|gid135=8+1217|gid203=8+0|gid26=13+1130|gid70=13+1006]

hb-shape --font-funcs=ft /Users/simon/Library/Fonts/lannaalif-v1-03.ttf

[gid237=0+2107|gid260=0+958|gid211=2+2348|gid260=2+958|gid201=4+1240|gid259=4+0|gid99=6+2435|gid229=8+2184|three=8 at -954,41+10|gid189=8+1200|gid260=8+958|gid189=8+1200|gid166=8+0|gid193=13+1319]

[*] Since you probably can't see that, here's the Unicode decomposition:
1a4b 1a63 1a31 1a63 1a27 1a62 1a20 1a55 1a43 1a76 1a63 1a60 1a36 1a36 1a63
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