#16908 has added the ability to callQuadmesh.set_array with a 2-D argument.
When a Quadmesh is instantiated, its grid of vertices is fixed. It would be good to
haveset_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 topcolormesh,
its dimensions are consistent with the dimensions of the vertex grid.