Round gvar and HVAR deltas the same in the glyf scaler

[dfrg]

This is more inconsistency where FreeType truncates the fractional bits when reading HVAR but reads gvar as Fixed and then rounds to F26Dot6.

https://github.com/googlefonts/fontations/blob/c32c0a6442371d0206f82a4a981b3854f313aaae/skrifa/src/outline/glyf/mod.rs#L628