The CSS filters feature is already enabled when unstable features and accelerated compositing is enabled and the acceleration backend is OpenGL. Layout tests should be unskipped as well.
CSS filters is enabled now on all ports since bug 136529.
I'm unskipping all the tests and retitling this bug and assigning here the detected failures (detected by manually checking the produced images on GTK versus Mac port).
css3/filters/effect-brightness.html [ Failure ]
css3/filters/effect-brightness-hw.html [ Failure ]
css3/filters/effect-brightness-clamping-hw.html [ Failure ]
css3/filters/effect-brightness-clamping.html [ Failure ]
css3/filters/effect-drop-shadow-hw.html [ Failure ]
css3/filters/effect-drop-shadow.html [ Failure ]
fast/filter-image/filter-image-svg.html [ ImageOnlyFailure ]
Committed r214292: <http://trac.webkit.org/changeset/214292>