Blend modes for layers

[philippotto]

For some use cases it would be helpful to have Photoshop-like blending modes for each layer. This would require that the order of the layers can be changed.

See here for an overview of blending modes in GIMP including the mathematical formulars.

Things to take care of:

• the changes should be backwards compatible (existing datasets should render exactly as before, even though the order of the layers is taken into account <-- maybe make the layers "additive" by default?)
• use case 1: blend high res and low res data in a "sensible way"
• use case 2: allow to achieve something like an alpha channel (e.g., everything which is black should be invisible by using a "Screen" blending mode)