Sorry about the delay.
It seems to me we could have some tests for this. e.g. create a font with an identity world transform, measure its metrics and some of its glyph widths, then add a transform and use the identity metrics/widths to compute the expected metrics/widths based on the new transform.
Also, if we're going to do some function renaming, could we go from e.g. INTERNAL_YDSTOWS to the vastly more readable height_to_LP rather than the other way around? Could we please do this renaming in a separate commit, so that the actual changes end up being in a smaller commit?