graphs/gradientmap: Support multiple stops
graphs/gradientmap: Allow to specify srgb colorspace
graphs/gradientmap: Add opacity property

Controls how much the gradientmap effect is blended into the original image.
One problem is that the blend op does not respect the "srgb" property yet,
because we have no way of sending the same input property to two nodes. See #58
tests: Update expected value to reflect property additions in graph
dependencies: Update with improved gradient-map operation