fix [#35453] "copy mirrored uv coords" doesn't work

- made precision configurable.
- report a warning when doubles are found since they cause problems.

added Polygon.center attribute to avoid calculating in python.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@57244 954f8c5b-7b00-dc11-b283-0030488c597c