[spot] alternate metrics ignored in class kern proof

[takaakifuji]

We found that some kerning pairs in CJK fonts were printed with no palt applied when generating a PostScript proof with $ spot -P palt,kern font.otf.

• Steps to reproduce
  1. Grab IBMPlexSansJP-Regular.otf, which comes with class-based kana kerning pairs along with palt/vpal.
  2. Generate a PostScript proof by running $ spot -P palt,kern IBMPlexSansJP-Regular.otf > IBMPlexSansJP-Regular.ps.
  3. Go to p.64 and look for ぐ (CID+857) and 。 (CID+635) pair.
• Expected result
  • For the pair ぐ。, the alternate metrics for both of the characters should be applied.
• Actual result
  • For the pair ぐ。, the alternate metrics for both of the characters are not applied.

See the following screenshots for better illustration:

Expected	Actual

For reference, the following is the result obtained with hb-shape:

$ hb-shape --features='palt,kern' IBMPlexSansJP-Regular.otf 'ぐ。'
[gid857=0@-105,0+785|gid635=1+500]

Applying a patch like this to c/spot/source/GPOS.c should fix this issue, so my speculation is that the code for handling palt is somehow missing for class kerning.