PolyStrips: Displaying and adjusting corners

[jlampel]

Handles in PS v4 are currently drawn at the start of the interpolated curves, while in v3 they were drawn at the center of the face. While v4 feels more accurate, v3 looks much cleaner.

Beyond just looking cleaner though, adjusting a corner in v3 adjusted both strips at the same time, while adjusting a corner in v4 adjusts only one and is disconnected from the other.

v3	v4

It would be great to have the same visualization and functionality in v4 if possible.