Only write the px value in Show for Au

The integer Au value is mostly internal and does not make much sense.

Printing both values is very verbose and hurts readability of debug logs.
Dump display lists when debug!() is enabled for gfx::display_list
Convert back to physical some layout coordinates.
Set the origin of the root element per its writing mode.

It’s not always (0, 0) anymore.
Reftests can opt into --experimental
Add a reftest for basic block layout in a vertical writing mode.
Merge pull request #3110 from SimonSapin/writing-modes-reftest

Add a basic CSS Writing Modes reftest