Attempt to fix incremental layout

I have no idea how to test it,
but this code builds and is close enough to what it was befor #1109.

Review much needed.
Remove (now unused) NetSurf’s libcss and libwapcaplet.

This removes four submodules (two C libraries and their Rust bindings.)
Remove the 'border-width: 0' hack.

This was to work-around a netsurfcss quirk.
auto merge of #1112 : SimonSapin/servo/newnewcss, r=metajack

Following up on #1109. These three commits are independent.