* app/gegl/gimpoperationlayermode.c: Make layer modes work on
premultiplied data. This makes compositing 40% faster on my test
image with 10 interlaced Normal and Addition mode layers.

* app/gegl/gimpoperationadditionmode.c: Switch to the
premultiplied algorithm and remove the experimental ones. The new
Addition mode has two important differences over the legacy
Addition mode:

  o Addition mode now really is commutative as the GIMP
    documentation says (this wasn't previously the case).

  o Layers in Addition mode are just as opaque as Normal
    layers. That is, their effect on the composite alpha channel
    is the same. For discussion, refer to bug #387449.

svn path=/trunk/; revision=27396