Support "replaced" layout

[nicoburns]

What problem does this solve or what need does it fill?

"replaced" layout is the layout mode for "foreign" content like images, videos, form fields, iframes, etc. These elements act a little differently to regular elements, but have standard (or at least interoperable) algorithms for computing their size.

What solution would you like?

• A new display mode Display::Replaced
• 2 new style properties
  • intrinsic_aspect_ratio: Option<f32>
  • intrinsic_size: Size<Option<f32>>
• A new compute_replaced_layout function that computes that size of Display::Replaced nodes.
• Teach the test generator about replaced nodes (probably starting with images)

What alternative(s) have you considered?

• Making the replaced "styles" part of the NodeContext instead. I'm not sure if this would be a better design, but I think it would be more complicated to design.

Additional context

A significant amount of bugs in the Taffy integration with Servo are down to the interaction between Taffy algorithms and replaced nodes. I'm currently thinking that making replaced nodes a first-class concept in Taffy would be the best way to fix that.