Fix some writing mode mismatch assertions errors.

The rendering is still wrong beause of #2795, but at least we get a rendering.

(This test change is just for readability, it should be equivalent to before.)
auto merge of #4425 : SimonSapin/servo/mixed-writing-modes, r=mbrubeck

The rendering is still wrong beause of #2795, but at least we get a rendering.

(This test change is just for readability, it should be equivalent to before.)

r? @mbrubeck