[HarfBuzz] disabling Arabic shaping when direction is vertical

[Jonathan Kew]

Hi Behdad,

If the harfbuzz buffer direction is vertical (TTB or BTT), I think we 
should refrain from doing Arabic shaping.[1]

We could do this, for example, by adding an early return such as
   if (unlikely (HB_DIRECTION_IS_VERTICAL (buffer->props.direction)))
     return;
in setup_masks_arabic(), so that the shaping analysis and setting of 
per-glyph features doesn't happen. WDYT?

(It's less clear to me what should be done in Indic and similar shapers, 
so I'm ignoring that for now...)

JK


[1] See CSS Writing Modes, section 5.1.1 Vertical Typesetting and Font 
Features: "...characters from horizontal cursive scripts (such as 
Arabic) are shaped in their isolated forms when typeset upright".