Fix wrong splitting of text row

Fix wrong splitting of text row

The code that tries to decide whether it is worth splitting a given
text row element had a shortcoming: it did not take into account the
left margin of the new row that would be created.

The problem is that this left margin is not the same as the left
margin of the current row, because there can be for example
indentation effects.

To fix this problem, we pass the amount of available space on the
next row as a parameter of Row::shortenIfNeeded.

Note that there is no need to care about RtL row elements at this
point, since the bidi algorithm will be applied to the row
subsequently.