mesh_plot/quadmesh - set a "maxpixels" arg

[mdsumner]

Consider applying a maximum to how much work is going to occur, mesh_plot also needs to be aware that it may be adding to a plot - so the maximum may not make sense for what is going to be seen (and non-trivial relationship given projection).

A "preview_meshplot3d()" and "preview_meshplot2d()" might be sensible too, so it's clear you get a quick look by using those - the message/stop/warning from above should mention these.

ping @kimnewzealand for the idea