Updating colormap doesn't change visual representation

[jo-mueller]

Just as a place to continue the discussion following PR #42:

It is strange to me that when the colormap is updated, triggering the color_indices setter needs to be executed in order for the coloring to take effect

[zoccoler]

This may be related to #52 now.

Some properties directly call self.draw(), others set another property that calls self.draw() (your example) and other just don't.