Scrolling issues

[jrmuizel]

These are some of the issues that we ran into in Firefox:
http://keithclark.co.uk/articles/pure-css-parallax-websites/demo3/
https://bug1148582.bmoattachments.org/attachment.cgi?id=8585108

[mrobinson]

The second case is working well in Servo now, but the first one definitely has clipping issues.

[nical]

The second test case now works in gecko as well, the first one is still broken.

[glennw]

As far as I can tell, these are both working correctly in Gecko + WR now.

[staktrace]

When scrolling the first link on macOS firefox nightly, I see artifacting at certain points. For example at the moment where the "Base Layer" text aligns with the "Background Layer" text things go weird, with a rect of a different color showing up and some of the tabs backrounds going partially green. This looks an awful lot like the issues in #2133, so we should probably track it there.