FT_MulAdd_Fix should have rounding
I suspect the fact that the new implementation does not round the same way is the reason why it causes a Blink layout test to produce a slightly different result: https://chromium-review.googlesource.com/c/chromium/src/+/3737625 external/wpt/css/css-text-decor/text-underline-position-from-font-variable.html
We should reintroduce the same rounding as before.