folded line text gets cut off with :set rightleft

[bfredl]

Steps to reproduce:

• use vim master, huge build
• Start with vim -N -u NONE.
• write two lines of text and make a fold of them using zf.
• execute :set rightleft

Now the fold line text will have two issues:

• Text is not drawn right to left, even though the code in fold_line and text_to_screenline in part looks like it intends to do rightleft drawing (for instance the handling of arabic shape depends on w_p_rl)
• When reducing the window width, fold line text gets cut off and replaced by the fill char even though there is lots of space left. Roughly maximum half of the width seems to be used to show fold text.