code cleanup: BM_face_create_ngon()

Change some of the paranoid checks into an assert since they should never happen.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53522 954f8c5b-7b00-dc11-b283-0030488c597c
change limited dissolve angle limit default from 15 -> 5 deg. since 15deg is quite high for 2 surfaces to be considered co-planar


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53523 954f8c5b-7b00-dc11-b283-0030488c597c