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.
For `vertical-align: middle` and `vertical-align: text-bottom`, we used
just the content height of the inline box to determine its alignment
position. This caused incorrect positioning when padding is applied.
This fixes the button alignment on our GitHub page.
Fixes#290.
