Make column-span work for columns grandchildren

[fabiobatalha]

I did some tests to test the merge-request (#878)

This merge request is related to the issue: #719

I took the same article displayed in the issue #719 and tried to produce the PDF file.

No changes were noticed on the PDF preview.

WeasyPrint version: v50

The original files are available in the zip below:

column-span.zip

Expected result:

Got:

[Tontyna]

As the title of #878 announces: It's only a partial support.
Of course, column-span: all should span across all columns of the nearest multicol ancestor but at the moment it looks like it only manages to span across the columns of the parent element.

That's why the footnote's title spans across the page, but the figures and tables don't.

BTW: It's your lucky day that WeasyPrint ignores the figures and tables and doesnt crash in the footnote, see #984

[liZe]

Of course, column-span: all should span across all columns of the nearest multicol ancestor but at the moment it looks like it only manages to span across the columns of the parent element.

Exactly. The code currently looks at the children to find column-span: all, but not at their own children.

Changing this behavior is not that easy, because we don't want to check all the grandchildren (nested columns, for example), we probably have to take care about some CSS properties like display and position, and we'll have to split the children using a resume_at / skip_stack tuple instead of a simple index.