enable planar reflection in MinitaurGymEnv

enable follow cam in other Gym locomotion environments
add testing assets for multi-material obj files -> sdf conversion.
Also use ER_NO_SEGMENTATION_MASK flag for TinyRenderer/EGL plugin renderer

data/threecubes/three_cubes.mtl

@@ -0,0 +1,31 @@
+# Blender MTL File: 'None'
+# Material Count: 3
+
+newmtl Material
+Ns 92.156863
+Ka 0.000000 0.000000 0.000000
+Kd 0.640000 0.000000 0.000000
+Ks 0.500000 0.500000 0.500000
+Ni 1.000000
+d 1.000000
+illum 2
+
+
+newmtl Material.001
+Ns 92.156863
+Ka 0.000000 0.000000 0.000000
+Kd 0.000000 0.640000 0.000000
+Ks 0.500000 0.500000 0.500000
+Ni 1.000000
+d 1.000000
+illum 2
+
+newmtl Material.002
+Ns 92.156863
+Ka 0.000000 0.000000 0.000000
+Kd 0.000000 0.000000 0.640000
+Ks 0.500000 0.500000 0.500000
+Ni 1.000000
+d 1.000000
+illum 2
+