glyphsapp smart components with non-integer setting values can cause off-by-ones

In the glyphs UI when adjusting smart components with the slider it is easy to end up with non-integer values, like:

and when we are actually instantiating these smart components we are sometimes running into some subtle point differences when comparing with fontmake.

There is a minimal test case attached.

[Iansui-build-off-by-ones.glyphs.zip](https://github.com/user-attachments/files/24062909/Iansui-build-off-by-ones.glyphs.zip)

Start by reproducing the issue with the attached Iansui-build-off-by-ones.glyphs.zip minimal test case, focusing on smart-component instantiation with non-integer slider values. Compare the resulting points with fontmake; done means the subtle off-by-one differences no longer occur for this case.

Written by the indexing model from the issue text.