Do not force a direction before we know it

WebCore does not calculate the text direction when calculating widths
the best we can do here is let the shaper guess the direction. Forcing
it to the default LTR breaks HarfbuzzNG on Arabic scripts.

Since the default HarfBuzz engine changed in 5.4, this was a regression
from 5.3.

See also https://bugs.webkit.org/show_bug.cgi?id=101440

Task-number: QTBUG-41450
Change-Id: Ia324c8e5ebd1744ce2dfaea94d5046ef1f77b483
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>