allow pybullet_envs.deep_mimic.testrl --arg_file run_humanoid3d_backflip_args.txt to perform a backflip. Can only backflip twice, then drops on ground.

this deepmimic is still very slow, due to slow mass matrix/inverse dynamics computation. once spherical motor drive is enabled, it should be fast(er)
move pd_controller_stable to pybullet_utils for easier re-use
add plane_transparent.urdf to pybullet_data
allow spacebar in keyboardEvents (Windows for now)

examples/OpenGLWindow/Win32Window.cpp

@@ -67,6 +67,11 @@ int getSpecialKeyFromVirtualKeycode(int virtualKeyCode)
 
 	switch (virtualKeyCode)
 	{
+		case VK_SPACE:
+		{
+			keycode = B3G_SPACE;
+			break;
+		}
 		case VK_RETURN:
 		{
 			keycode = B3G_RETURN;