Improve masked regions preview

[bnmajor]

Pulled from the comments in a previous issue we should make the following improvements;

1. Make subplots use the same range for coloring. Right now subplots with masked regions have 1 as yellow and 0 as purple whereas unmasked images have 1 as purple
2. Support polar and/or stereograph views since this is how it is commonly presented at meetings, etc.
   1. Instead of raw use only polar? -OR-
   2. Ability to toggle between views like with image data? -OR-
   3. Use the view that matches the current image mode?

[psavery]

They sound good! I think that for 2, we can use the view that matches the current image mode.

However, since we don't support masking in the Cartesian view, maybe we should just display a raw view of the masks for that one? We can put in text at the top of the dialog what view is being used to generate the masks, for clarity.

One extra reason for doing it this way: we currently generate the threshold mask for the polar view after the polar view has been generated. So we would need the generated polar view to accurately display the threshold mask in the polar view.

[bnmajor]

This sounds great, thanks @psavery!