vf_drawtext: drop text wrapping feature

Or in other word, do not go to the next line when the text cannot be
rendered within the frame.

The rationale is that wrapping is hard to manage, as it depends on the
position of the text, thus making hard/impossible to compute the size
occupied by the rendered text, and makes the filter behavior
unpredictible with moving text (implemented in a pending patch).

Also the current wrapping code is very limited (for example it splits
words in the middle), and correctly handling wrapping requires much
more complexity (alignment, gravity, word parsing etc.), while
drawtext is meant to be only a thin wrapper around libfreetype, so
such advanced features don't belong here.
