Describe the issue

Summary

Right now, the coordinates of a Quadmesh, returned from pcolormesh, are not available to the end-user. They are stored internally in the coordinates attribute. This is an issue for Cartopy, when we need to know where the MPL calculated coordinates are located to know whether we should mask some cells for users.

Proposed fix

Add a .get_coordinates() method that returns the private attribute to Quadmesh. This would be minimally invasive, and only ensures that Cartopy isn't relying on a private attribute.

Related to #18317, and my proposed fix in SciTools/cartopy#1646 that relies on the private attribute.