Ad hoc meshing does not work for segmentations with mags >= 64

[philippotto]

By default, ad hoc meshing uses a cubeSize of [128, 128, 128] (when specified in mag 1). For mag 64, this corresponds to a cubeSize of [2, 2, 2] and with a padding then [3, 3, 3]. However, this seems to produce incorrect meshes for some reason.

As a workaround, one can run window.__marchingCubeSizeInMag1 = [4096, 4096, 4096] in the current tab. The default cube size doesn't seem well chosen for these coarse mags. Therefore, I think, we should adapt the front-end to send different cube sizes to the back-end.

However, I'm not 100% sure why small cube sizes don't work at all. Is it problematic if a bucket contains only one segment and then no mesh is returned for that bucket or something like that?