Webrender: Transformed primitive rendering glitches

[kvark]

See https://github.com/servo/webrender/pull/2069#issuecomment-347914944 for original report.
Related bugzilla entries:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1420748
• https://bugzilla.mozilla.org/show_bug.cgi?id=1420754
• https://bugzilla.mozilla.org/show_bug.cgi?id=1421526

[glennw]

@staktrace Thanks for tracking down the regression range. Self assigned for when I have time to look at this.

[staktrace]

While bug 1420748 appears to be fixed in the latest firefox nightly, the other two bugs are still happening. I'll open separate issues for them.