Color sampling

This pull request fixes the colour sampling in the ImageView to sample linear colours under the mouse rather than the post gamma/exposure/lut colours. In doing so it refactors the ImageView a little in the direction I would like it to go, using a new ImageSampler node internally to do the sampling - the next phase is to hook the ImageSampler outputs up to PlugValueWidgets in an overlay and do the drawing that way, removing much of the additional code in ImageView.

This goes some way to addressing #663, but isn't enough to close it.