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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>