Inline nodes in our layout tree have a position, so let's show it. By
centralizing the logic for this, block nodes now lose their redundant
'content-size' dump info which is already part of the box model dump.
This suits the spec a bit better, and exposes the fact that we were
allowing `::ImageButton` to use the button layout although it is never
specified that it should do so. Tests were rebaselined for this.
Browsers such as Chrome and Firefox apply an arbitrary scale to the
current font size if `normal` is used for `line-height`. Firefox uses
1.2 while Chrome uses 1.15. Let's go with the latter for now, it's
relatively easy to change if we ever want to go back on that decision.
This also requires updating the expectations for a lot of layout tests.
The upside of this is that it's a bit easier to compare our layout
results to other browsers', especially Chrome.
This implements enough to represent <input type=image> with its loaded
source image (or fallback to its alt text, if applicable). This does not
implement acquring coordinates from user-activated click events on the
image.
