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/threecubes.py

@@ -0,0 +1,8 @@
+import pybullet as p
+p.connect(p.DIRECT)
+p.loadPlugin("eglRendererPlugin")
+p.loadSDF("newsdf.sdf")
+while (1):
+	p.getCameraImage(320,240, flags=p.ER_NO_SEGMENTATION_MASK)
+	p.stepSimulation()
+