Floating outside the parent box makes surrounding text leak

[Smylers]

Applying a margin to a floated element can make text that flows round it leak outside its box,if the floated element fits entirely outside the box. See http://www.stripey.com/demo/weasyprint/float_outside_box.html and http://www.stripey.com/demo/weasyprint/float_outside_box.pdf

Each is floated left and has a negative left margin, so it starts left of its parent <p>. In the first paragraph the text of the <span> extends far enough back into the <p>, and the rest of the <p> text flows round it, as expected.

In the second paragraph the <span> text is short enough to fit entirely outside the content area of the <p>; the rest of the text in the <p> can avoid it simply by staying inside the <p>. This is what happens in Firefox and Chrome. But in Weasyprint the first line of the following text starts immediately after the right edge of the <span>, left of its own content area; it's moved into the negative margin that should only apply to the <span>.

[SimonSapin]

This is a bug in the handling of "inline floats". A max() or something is missing in weasyprint.layout.inlines.split_inline_box().

[liZe]

Fixed in 6e114af, but more cases are broken (see issue #39).