Fix AssertionError in layout.inlines.split_text_box().

[natano]

Traceback (most recent call last):
  [...]
  File "/app/env/lib/python3.5/site-packages/weasyprint/layout/inlines.py", line 718, in split_text_box
    'Expected nothing or a preserved line break' % (between,))
AssertionError: Got '1,' between two lines. Expected nothing or a preserved line break

The Assertion Error can be triggered with following minimal test case
(Adobe's Source Sans Pro font must be installed):

<style type="text/css">
    p {
	font-family: 'Source Sans Pro';
	font-size: 24pt;
	width: 275pt;
	overflow-wrap: break-word;
    }
</style>
<p>W1D1,W1D7,W2D14,W3D21,W4D28</p>

With the Adobe Source Sans Font the pango line wrapping algorithm
sometimes produces sporadic results. The wrapping seems to be dependent
on the following text, so that a short text doesn't "fit" on a line, but
does if it is followed by more text. This can be worked around in the
split_first_line() function by computing the resume offset at a later
point, so it is in sync with the actual wrapping behaviour.

See https://bugzilla.gnome.org/show_bug.cgi?id=777093.

[liZe]

Thanks a lot for reporting this issue!

Your fix is OK. We have to take the final second line index for resume_at because the temporary second line is only split on words (the second one splits on chars, that's how WRAP_WORD_CHAR works).

I think that we can safely use WRAP_CHAR instead of WRAP_WORD_CHAR as we tried to split the line on words boundaries earlier, and avoid a second split. I'll do this in another commit.

[liZe]

I think that we can safely use WRAP_CHAR instead of WRAP_WORD_CHAR as we tried to split the line on words boundaries earlier, and avoid a second split. I'll do this in another commit.

It doesn't work. I've added some comments about this.

Thanks again!