Extend ITriangleSelector::getTriangles() to allow for returning multiple groups of triangles from individual scene nodes.  By default, the 1.5 behaviour will be preserved, and all triangles from all nodes will be returned, even for IMetaTriangleSelectors.  Regression tests confirm this.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2124 dfc29bdd-3216-0410-991c-e03cc46cb475