Quite a lot of the example canvases use the trick of blending a transparent shape...

Quite a lot of the example canvases use the trick of blending a transparent shape using blend method "straight", and relying on an old bug to mask just the pixels in the shape.  Now that "straight" blending no longer only applies to the pixels in the shape, but to the whole layer, this trick no longer works.  The example .sif files have been modified to no longer rely on the trick, but it's possible that other .sif files do use it, and so this change looks out for the trick and changes it to use a fully opaque shape with "alpha over" blending, which has the same effect.

git-svn-id: http://svn.voria.com/code@1457 1f10aa63-cdf2-0310-b900-c93c546f37ac