Blender Git Loki

Calculate epsilon values for interp_weights_poly to improve accuracy

interp_weights_poly_v2 would have too large epsilon values for small
polygons. To solve this we now calculate the appropriate epsilon value
so it can gracefully handle big and small values.

To make sure there was no regression, these changes were tested with the
files in T36105, T31581. Also with a surface deform modifier test file
attached in the differential below.

Reviewed By: Brecht

Differential Revision: https://developer.blender.org/D7772