[HarfBuzz] application of hangul *jmo features to broken syllables

[Jonathan Kew]

The Hangul shaper should NOT apply the *jmo features to glyphs that are 
not part of a properly-structured Korean syllable.

Some examples of current behavior:

[GOOD: complete LVT sequence, proper features applied]
hb-unicode-encode 1101,1161,11f0 | hb-shape UnBatang_0613.ttf
[uni1101.ljmo01=0+1024|uni1161.vjmo01=1+0|uni11F0.tjmo01=2+0]

[GOOD: lone L does not have features applied]
hb-unicode-encode 1101 | hb-shape UnBatang_0613.ttf
[uni1101=0+1024]

[GOOD: LT without V is not valid, don't apply features]
hb-unicode-encode 1101,11f0 | hb-shape UnBatang_0613.ttf
[uni1101=0+1024|uni11F0=1+0]

[GOOD: lone T, don't apply features]
hb-unicode-encode 11f0 | hb-shape UnBatang_0613.ttf
[uni11F0=0+0]

[BAD: lone V, shouldn't apply vjmo]
hb-unicode-encode 1161 | hb-shape UnBatang_0613.ttf
[uni1161.vjmo02=0+0]

[BAD: VT pair without L is not valid, shouldn't apply *jmo]
hb-unicode-encode 1161,11f0 | hb-shape UnBatang_0613.ttf
[uni1161.vjmo01=0+0|uni11F0.tjmo01=1+0]

Note the last two examples; these are incorrect, IMO. In both these 
cases, Uniscribe does not apply the *jmo features.

JK