Add an rgb24 ref image for alpha similar

This commit copies the current alpha-similar.image.rgb24.out.png to
alpha-similar.rgb24.ref.png to make the test pass.

This failure goes back to commits 6647511 / 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>

.gitlab-ci/ignore-image-rgb24.txt

@@ -1,5 +1,4 @@
 a1-clip-fill-rule
-alpha-similar
 big-empty-box
 big-empty-triangle
 big-little-box

.gitlab-ci/ignore-image16-rgb24.txt

@@ -2,7 +2,6 @@ a1-bug
 a1-clip-fill-rule
 a1-tiger
 aliasing
-alpha-similar
 arc-direction
 arc-looping-dash
 big-empty-box