Fix cursorToX for right to left text with trailing whitespace.

Fix cursorToX for right to left text with trailing whitespace.

QTextLine::cursorToX returned the line width for cursor positions
outside the width of a wrapped right to left line because the
leading space width was always calculated as 0.

Returning a non-zero width for the leading space does cause
problems for other uses of QTextEngine::alignLine() though
as the textAdvance already doesn't include the leading/trailing
space so subtracting it there double accounts for it.

Task-number: QTBUG-24801

Change-Id: I56cbb139814c32813bebb49de8c045b29154a958
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>