Skip ear cutting algorithm for quadrilaterals, makes triangulation MUCH, MUCH faster (100x) for models composed of quadrilaterals (i.e. Blender, Terragen).

git-svn-id: https://assimp.svn.sourceforge.net/svnroot/assimp/trunk@773 67173fc5-114c-0410-ac8e-9d2fd5bffc1f