GS VF GPOS table build sizes: fontc vs. fontmake

I took a look at our GS Python pipeline vs. Rust pipeline build sizes today and the current Roman GS VF is ~3.5MB larger with fontc than it is with fontmake. It looks like the OT table with the most significant increase is GPOS at ~4x the size.

fontmake[pathops]==3.7.1 (PyPI release)
fontc main branch commit as of ~17h ago

Both executed in GH Actions Ubuntu linux environments. cPy 3.12 interpreter on the fontmake build. rustc stable toolchain with cargo release profile on the fontc compiler compile.

| | fontmake | fontc |
| ------------- | ------------- |------------|
| Total | 4836292 B | 8544752 B |
| GPOS | 1035870 B | 4352928 B |

I shared repro builds, runner logs, and full OT table size comparisons with Colin privately.