Eye_rigger: Fix for tilted- or asian eyes

[ivogrig]

Currently the middle vertices for the upper- and lower curves are
required to be the ones with the highest/lowest Y-value.

With a tilted eye this yields unexpected behavior (breaks).
This is an attempt to fix that issue.

[ivogrig]

I just realised that this solution is kind of assuming a particular eye-shape again and the closest-point would not work with donald duck-eyes either.
So the best fix would actually be to let the user manually override the mid-points through the GUI

[miquelcampos]

Hello @ivogrig ,

Thanks for the pull request. I will check this ASAP :)

one question: using the custom eye corners, did work with your model?