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)

Edited Jun 28, 2019 by Guido Gunther

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