Create a software filter like "blur" or "opacity" and wrap that in a composited layer. Make sure that they are the same size. The compositor will consider the layers to be opaque and thus render without opacity.
Created attachment 197314 [details] Patch V1
Comment on attachment 197314 [details] Patch V1 Attachment 197314 [details] did not pass efl-ews (efl): Output: http://webkit-commit-queue.appspot.com/results/17630042
Created attachment 197348 [details] Patch V2 Added missing ifdef.
Comment on attachment 197348 [details] Patch V2 Clearing flags on attachment: 197348 Committed r148117: <http://trac.webkit.org/changeset/148117>
All reviewed patches have been landed. Closing bug.
Could this have broken this test? http://build.webkit.org/results/Apple%20MountainLion%20Debug%20WK2%20(Tests)/r148117%20(8720)/results.html