spacing_top/spacing_bottom in fallback FontVariation does not change baseline

[gu-fan]

Tested versions

4.2.1.stable

System information

Linux

Issue description

When a FontVariation has a FontVariation in it as a fallback font

changing spacing_glyph in subresource seems does working after test as #80907 descripted.

But changing spacing_top or spacing_bottom in subresource does not working.

which I thought changing spacing_bottom/spacing_top will change the position of fallback font, so I can align the fallback font with main font by baseline.

I don't know if this is a bug, or just as it is.
and a new align baseline for fallback font feature need to be implemented.

Steps to reproduce

1. Create a Label.
2. Create and set a FontVariation as the font of the label node.
3. Create and set another FontVariation as a fallback font of the font of the label node.
4. changing fallback font's spacing_top and spacing_bottom takes no effect. but spacing_glyph works.

Minimal reproduction project (MRP)

bug_font_fallback.zip

[bruvzg]

It's not a bug, top and bottom spacing should not affect baseline in any way, it's added to ascent/descent, and intended for line spacing control, baseline always remain the same. But I guess baseline offset can be added as an extra property.

[gu-fan]

yes, baseline offset looks good