[css-ruby] Space distribution in situations with spanning annotations

Currently it is undefined that how ruby-align should be applied if there
are spanning annotations.

When previously discussed off the list, fantasai said that the behavior
would be equivalent to nested markup. However, I don't think it is
workable. With nested markup, there is a unique ruby-align value applied to
the whole inner ruby, while we may have different values for each level
when handling spanning. On the other hand, in cases like
<rbc><rb>a</rb></rbc><rtc>xyz</rtc>, whether the content in the rtc is
recognized as a span could affect the result, which does not look correct
to me.

I suggest that, in those situations, non-spanning pairs are directly
expanded to match the width of the whole ruby, as if they have a longer
annotation in some level, regardless of ruby-align. This could solve both
problems I mentioned above.

- Xidorn