Since bug 651469, Xlib Surfaces no longer depend on XCopyArea semantics to support self copies. Cairo used XCopyArea only when cairo created the pixmap itself, but cairo always initializes Pixmaps that it creates. Now that we don't depend on XCopyArea, gfxXlibSurface::CreateSimilarSurface can use gfxXlibSurface::Create which skips the black fill. (I haven't checked whether Gecko relies on the transparent fill for COLOR_ALPHA surfaces.)
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