Improve one-point-per-pixel logic.

[jsoules]

Adjust the observed-position overlay drawing logic so that it only skips points that would actually map to the same pixel.

Adjust the discontinuity-preservation logic so that it depends on the pixel distance between animal positions, rather than a percentage of the track. (This seems like the right thing to do for now, but it may prove problematic in the future, so we might need to back that part out at a later date.)

[jsoules]

(I mean specifically it would be a problem if the canvas was very tall.)

[magland]

Looks great, merging.