The contrast() filter does not deal with premultiplied alpha correctly

[mstange]

Testcase and more detailed description coming tomorrow.

[mstange]

https://bugzilla.mozilla.org/show_bug.cgi?id=1408116

Testcase

Expected vs. actual rendering:

I think it's worth making all color transform filters work the same: unpremultiply, apply a matrix, premultiply. Here's the Gecko code that computes the color matrix for the different filter types: http://searchfox.org/mozilla-central/rev/31606bbabc50b08895d843b9f5f3da938ccdfbbf/gfx/src/FilterSupport.cpp#396-513

[Darkspirit]

This got fixed with the latest webrender update. (8a39cf2...d490a74)
https://bugzilla.mozilla.org/show_bug.cgi?id=1408116#c2

[glennw]

Thanks!

[mstange]

This was fixed by #2048.