Needed for shader compile bustage on AMD cards on Windows - https://github.com/servo/webrender/commit/42af5d25d3d75357b6a1b4617368871b74cf6215

I did a WR update locally with the new process (see bug 1328910) and pushed to try along with some of Jerry's fixes. I'm seeing reftest failures locally and I expect to see the same reftest failures on try. Jerry said he'd follow up with Glenn about the failures, since he bisected the problem down to https://github.com/servo/webrender/pull/648 https://treeherder.mozilla.org/#/jobs?repo=try&revision=ff64d9b089b2853708ef779ca2fb9945e718c351

Forward-duping to the new bug Jerry filed. Since [1] landed after [2], we can't update to [1] before resolving the z-buffer breakage anyway, which Jerry is doing in the other bug. [1] https://github.com/servo/webrender/commit/42af5d25d3d75357b6a1b4617368871b74cf6215 [2] https://github.com/servo/webrender/commit/56131c91c5eb946a156af9ea06af86358aa77f4c