Add per-face smoothing groupd ids to mesh_t.

Add unit test for smoothing groups.

models/issue-162-smoothing-group.mtl

@@ -0,0 +1,23 @@
+newmtl test1
+  Ns 10.0000
+  Ni 1.5000
+  d 1.0000
+  Tr 0.0000
+  Tf 1.0000 1.0000 1.0000 
+  illum 2
+  Ka 0.0000 0.0000 0.0000
+  Kd 0.5 0.2 0.2
+  Ks 0.0000 0.0000 0.0000
+  Ke 0.0000 0.0000 0.0000
+
+ newmtl test2
+  Ns 10.0000
+  Ni 1.5000
+  d 1.0000
+  Tr 0.0000
+  Tf 1.0000 1.0000 1.0000 
+  illum 2
+  Ka 0.0000 0.0000 0.0000
+  Kd 0.2 0.5 0.2
+  Ks 0.0000 0.0000 0.0000
+  Ke 0.0000 0.0000 0.0000

models/issue-162-smoothing-group.obj

@@ -0,0 +1,51 @@
+
+# cube.obj
+#
+
+mtllib issue-162-smoothing-group.mtl
+
+v -0.500000 -0.500000 0.500000
+v 0.500000 -0.500000 0.500000
+v -0.500000 0.500000 0.500000
+v 0.500000 0.500000 0.500000
+v -0.500000 0.500000 -0.500000
+v 0.500000 0.500000 -0.500000
+v -0.500000 -0.500000 -0.500000
+v 0.500000 -0.500000 -0.500000
+
+vt 0.000000 0.000000
+vt 1.000000 0.000000
+vt 0.000000 1.000000
+vt 1.000000 1.000000
+
+vn 0.000000 0.000000 1.000000
+vn 0.000000 1.000000 0.000000
+vn 0.000000 0.000000 -1.000000
+vn 0.000000 -1.000000 0.000000
+vn 1.000000 0.000000 0.000000
+vn -1.000000 0.000000 0.000000
+
+usemtl test1
+g test1
+s 1
+f 1/1/1 2/2/1 3/3/1
+f 3/3/1 2/2/1 4/4/1
+
+usemtl test2
+g test2
+
+s off
+f 3/1/2 4/2/2 5/3/2
+f 5/3/2 4/2/2 6/4/2
+s 3
+f 5/4/3 6/3/3 7/2/3
+f 7/2/3 6/3/3 8/1/3
+s 4
+f 7/1/4 8/2/4 1/3/4
+f 1/3/4 8/2/4 2/4/4
+s 0
+f 2/1/5 8/2/5 4/3/5
+f 4/3/5 8/2/5 6/4/5
+s 6
+f 7/1/6 1/2/6 5/3/6
+f 5/3/6 1/2/6 3/4/6