refresh snapshots

[Fil]

This defaults the snapshots (mocha) to use offset=0 (as a follow-up to #1442).

And we refresh all snapshots, which also benefit from the reduced precision in numbers (#2270, #2234)

closes #1445

Additionally, working on this revealed a bug, where the image generated when offset=0.5 was using a different origin for the hexes. As a result, the image produced was different when the DPI offset changed; two viewers using different screens would not see the same binning. The image now is the same independently of the DPI offset (but its coordinates are shifted by -0.5px and the path is translated by +0.5px). Note that the images in the test snapshots have changed, since offset=0 is now our baseline, but that's OK.

The bug is now fixed, but this reveals another issue: I've tried to create a new snapshot test that would show why we need ox=0.5 in hexbin.js, but I can't understand the logic any more, since what we gained on the right-hand-side by having the outer hexes be "inside" the frame, we lost on the left-hand side.

ox=0	ox=0.5
bad on the rhs	bad on the lhs

(A better solution would be to rescale around the center by a factor 0.999999999…? which I'm doing here #2367)

[Fil]

Investigating the 6 line difference that this caused in walmart.html, I see that because of a mix-up between offsets (the DPI offset and the hex offsets), we ended up having a different image with the DPI offset compared to without. This is a bug: while the rounding is obviously a bit arbitrary, the offset should only translate the rendering, not modify the binning.