When zooming in too far, no data is rendered

[philippotto]

If the viewport covers less than 32 voxels in one dimension, no data is rendered. I think, this is a regression which was introduced in 1fb0ec3#diff-92d2231c9c52b0b0ecc939eb4f576b42684b06d0d393b45244467e2a7873fec5R92.

@daniel-wer just fyi. i think, it's not urgent since one has to zoom in really, really far to run into this. maybe the bucket picker shouldn't even be called if a viewport is invisible..