Add an option to validate display list items

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.
auto merge of #3747 : mrobinson/servo/validate-display-list, r=pcwalton

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.