Add missing ref image for clip-unbounded
This commit copies clip-unbounded.base.rgb24.ref.png to clip-unbounded.rgb24.ref.png. This makes the test pass for rgb24 targets.
This failure goes back to commits 66475115 / 049060 which changed the image comparison in the test suite. Previously, transparent "nothing" and black compared equal, but said commits fixed that. Thus, having a special reference image with black instead of transparency is the right fix.
Signed-off-by: Uli Schlachter psychon@znc.in