Black screen since PR 184

[paulrouget]

Since #184, during the tab switching animation, browser.html turns black (osx).

[pcwalton]

Huh, I'm pretty sure I tested that animation. Perhaps I messed up when preparing the patch.

I suspect this will show up as a GL error in apitrace if I can reproduce. I had some issues in the patch with trying to use textures that were deleted that I had to fix. I bet it's something like that.

[paulrouget]

http://people.mozilla.org/~prouget/servo-apitrace-2016-02-17.trace

[paulrouget]

I see no error. But I'm not familiar with opengl and apitrace.

[glennw]

@paulrouget Do you want us to back that change out (it's a performance fix only) until @pcwalton is able to repro it, or is it not impacting your work much?

[paulrouget]

Not blocking us. We can wait.