This is quite a bit cleaner than abusing the rust debug functionality.
If we start collecting too many debugging options in the servo
executable we could opt to organize them into a single option.
Fixes#2263.
When this option is enabled, the layout task will print an error when
display list items draw outside their owning Flow's position rect. This
will make it easier to detect layout errors before they break rendering.
This is a command-line option for the moment, because we violate this
rule quite a bit still. Once all bugs causing this are fixed, we can be
more aggressive about enabling the option.
Note that using `servo --devtools http://example.org` doesn't work. In
that case either the port must be specified or the option moved to the
end. This is done by getopts and is the same for other such options,
e.g. `--profile`.
