Fix pixel snapping code - this now passes the border rounding tests, and all the related css tests.
Fix pixel snapping code - this now passes the border rounding tests, and all the related css tests.

It does expose bugs in a few other tests / layout code areas, but individual patches are coming for those.

servo/webrender,servo/webrender,servo/webrender,servo/webrender,alanamramjit/webrender,servo/webrender,servo/webrender,alanamramjit/webrender