#16908 has added the ability to call Quadmesh.set_array with a 2-D argument.
When a Quadmesh is instantiated, its grid of vertices is fixed. It would be good to
have set_array validate its argument so that regardless of whether it has already
been flattened or whether it is a 2-D array such as one would supply to pcolormesh,
its dimensions are consistent with the dimensions of the vertex grid.