extend Wavefront obj loader to recognize objects (for compound shape creation)

add reproduction of a bug in compound versus concave trimesh

data/cube.mtl

@@ -0,0 +1,11 @@
+# Blender MTL File: 'None'
+# Material Count: 1
+
+newmtl Material
+Ns 96.078431
+Ka 0.000000 0.000000 0.000000
+Kd 0.640000 0.640000 0.640000
+Ks 0.500000 0.500000 0.500000
+Ni 1.000000
+d 1.000000
+illum 2

data/cube.obj

@@ -0,0 +1,26 @@
+# Blender v2.66 (sub 1) OBJ File: ''
+# www.blender.org
+mtllib cube.mtl
+o Cube
+v 1.000000 -1.000000 -1.000000
+v 1.000000 -1.000000 1.000000
+v -1.000000 -1.000000 1.000000
+v -1.000000 -1.000000 -1.000000
+v 1.000000 1.000000 -0.999999
+v 0.999999 1.000000 1.000001
+v -1.000000 1.000000 1.000000
+v -1.000000 1.000000 -1.000000
+usemtl Material
+s off
+f 1 2 3
+f 5 8 7
+f 1 5 6
+f 2 6 3
+f 3 7 4
+f 5 1 4
+f 4 1 3
+f 6 5 7
+f 2 1 6
+f 6 7 3
+f 7 8 4
+f 8 5 4