[gvar] we could allocate less when building tuple variations

Choosing the correct packed points format for a given tuple requires us to fully build that tuple twice, after which we throw away these results; we then build the tuple a third time, at the end. In theory we could have some shared set of buffers that we reuse for all of this work, which would be slightly more complicated.

In practice this is very low priority, but something to look at as directed by profiling.

Start by profiling the gvar tuple-variation building path described in the issue, measuring the repeated tuple construction and discarded results. Trace where the packed points format is selected and where the tuple is built a third time. Done means shared buffers reduce allocation or construction work while preserving the generated font data and existing behavior.

Written by the indexing model from the issue text.