py,ui: revert to old behavior of view transition blend

This works in most cases, but instead of the whole group first getting composited and then adjusted it is the alpha of all individual draw calls that are tweaked through the global alpha.

Issue #122 (closed), I am not sure compositing groups have ever worked properly - it is a very little tested feature. The API was used wrong anyways the global opacity changed the opacity of all render sources in the incoming view to have bits of transparency, so there might have been no pixel compositing going on, or if it was it was on pre-made-transparent bits instead of a texture being made transparent during compositing.