Bring back alpha blending
Alphablending got lost since the switch to layer-surfaces. See be400ceb231a02a3d303e0a044b0d5e0d45112b2 of our old phosh branch. The code to use an alpha channel on surfaces is merged upstream (see e.g. the layer-surface demo).
this can sometimes be useful in addition to the sliding animations in #27 (closed)