ImagePlot interpolation will need to change if/when celiagg is adopted by Enable

[jwiggins]

This is the current implementation for setting the interpolation mode when drawing an image: https://github.com/enthought/chaco/blob/cd191bc4ab3fb902c20b49dd604ad8d63cc31b3c/chaco/image_plot.py#L194-L204

If/when we switch to celiagg as the default backend for Enable, this will need to change. Image interpolation will instead be set on the GC.