Followup fix for 3f9e2dced298c3d00f31b2dfc38685bb071a3a22

During GSUB, if a ligation happens, subsequence context input matching
matches the new indexing.  During GPOS however, the indices never
change.  So just go one by one.

Fixes 'dist' positioning with mmrtext.ttf and the following sequence:

  U+1014,U+1039,U+1011,U+1014,U+1039,U+1011,U+1014,U+1039,U+1011

Reported by Jonathan Kew.