enable transparent background images (ticket #233)


git-svn-id: https://geuz.org/svn/gmsh/trunk@18540 59c8cabf-10d6-4a28-b43f-3936b85956c4
pp


git-svn-id: https://geuz.org/svn/gmsh/trunk@18541 59c8cabf-10d6-4a28-b43f-3936b85956c4