The comment said that using CAIRO_OPERATOR_SOURCE for the background
triggered a librsvg bug, but the relevant commit message does not even
include a link to a librsvg bug.
Also, changing it from OVER to SOURCE completely breaks these
tests (the reference images don't match at all), so this comment is
stale. Just remove it.
Signed-off-by: Bryce Harrington <b.harrington@samsung.com>