mesh_plot needs to catch attempt to transform to crs when projection(x) is not defined and coords is given

[mdsumner]

e.g. in angstroms

mesh_plot(temp0[[2]], crs = projection(boxes))

temp0 has no CRS, so coords must be supplied

[mdsumner]

Merge the logic in quadmesh() itself into a separate function and replace for mesh_plot

[mdsumner]

target_coordinates() already has the logic, just allowed it to shortcut return when no projection required

[mdsumner]

Sweet, all done!