After restarting Portal, the problem was gone, so I refrained from updating my
Mesa :-)
-Before and after images:
+Before and after images (probably Copyright by Valve, but I consider this to be
+fair use):
<div class="gallery">
[[!img testchmb_a_000000.jpg alt="Before"]]
[[!img testchmb_a_000002.jpg alt="After"]]