Speed up generation of animations by reusing PixelMaps

[JonBlower]

At the moment, if an animation is requested, a PixelMap is generated for each frame of the animation. This is wasteful, particularly in cases where generating the PixelMap is expensive (e.g. a curvilinear source grid).

A mechanism could be found to generated the PM only once and use it to extract the data for each frame. This may involve changing the level of abstraction of the relevant Layer.read() method. Alternatively the PixelMaps could be cached, avoiding the expense of regenerating them (see #137).