Investigate possible regression in handling of fractional negative X1/Y1 crop coordinates

The behavior of negative values for X1/Y1 is not defined by our docs, but X2/Y2 values of zero or less are bottom/right relative. 

We need to verify that V4 did not affect how we interpret negative X1/Y1 values.
https://github.com/imazen/resizer/blob/develop/Core/Util/PolygonMath.cs#L782-L786

There is a possibility that we could easily change how we interpret X1/Y1 values -0.5 <= x1/y1 <= 0. 
