Add support for two eyes

[tmalsburg]

and take advantage of the fact that it's only a saccade if it happens in both eyes. This way we may also solve the problem of consecutive samples whose velocity dances around the threshold.

Left eye:   __XXX_XXXXX____
Right eye:  ___XXXXX_XXX___
Saccade:    __XXXXXXXXXX___

Just use an OR to combine the two saccade vectors. Smoothing the signal or the velocities may not any longer be necessary.