When layouting a replaced element with natural width and height (e.g. a
raster graphic), the replaced element would correctly end up with its
natural size in the main-axis dimension. For the cross axis dimension
however, the replaced element was stretched or squished to the flex
containers inner cross size, which is wrong. Instead, we need to respect
the replaced elements aspect ratio.
Since the touched code does not have a direct correspondence to any spec
text, I am not fully certain that the change is completely correct.
However, tests agree with it, so the new code seems more correct than
the old one at least.
This fixes 50 WPT subtests in `css/css-flexbox`, most of which are
already in-tree. I have also created a new test for a scenario that did
not seem to be covered by WPT.
