Wanted: bring back grat_mask feature

[Maschette]

We used to have the ability to crop an automap object to a graticule which was disabled (0193e9c#diff-e1e1d3d40573127e9ee0480caf1283d6 ) if there is no reason not to have it I would like to bring it back.

@mdsumner is there any reason the old code wouldn't work anymore?

[Maschette]

Is this what fast_mask was written for?

[mdsumner]

It's not easy. I think we should do a few examples manually and see how that goes.