Correct fix for r54164, the testcase I was using worked but different edge slide examples didn't.

Edge slide needed to check for concave ngons too.

add BM_loop_is_convex() and expose to python too.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54173 954f8c5b-7b00-dc11-b283-0030488c597c