Auto merge of #2491 - kvark:flicker-workaround, r=glennw
Auto merge of #2491 - kvark:flicker-workaround, r=glennw

textureSize() workaround for Angle/NV

Temporary workaround for flickering issues of https://bugzilla.mozilla.org/show_bug.cgi?id=1431560

It is not covering all the cases, but on the other hand is rather harmless. Should ease the pain of Windows/Nvidia users while the proper cure (my Angle fix) is in flight. Check out [this comment](https://chromium-review.googlesource.com/c/angle/angle/+/949412#message-fe4f62522349e7754d545ffc8e96c999d3192d26) for nasty technical details.

WR driver bug entry: https://github.com/servo/webrender/wiki/Driver-issues#texturesize-in-vertex-shaders

r? @glennw

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